Add unshare to podman

This command lets the user run a command in a new user namespace like `unshare -u`.
It uses the implementation of unshare in buildah. ( fixes #1388 )

Signed-off-by: Divyansh Kamboj <kambojdivyansh2000@gmail.com>

2 files changed, 90 insertions, 52 deletions

diff --git a/docs/podman-unshare.1.md b/docs/podman-unshare.1.md
new file mode 100644
index 000000000..a7f018ce1
--- /dev/null
+++ b/docs/podman-unshare.1.md
@@ -0,0 +1,37 @@
+% podman-unshare "1"
+
+## NAME
+podman\-unshare - Run a command inside of a modified user namespace.
+
+## SYNOPSIS
+**podman unshare** [*options*] [**--**] [*command*]
+
+## DESCRIPTION
+Launches a process (by default, *$SHELL*) in a new user namespace. The user
+namespace is configured so that the invoking user's UID and primary GID appear
+to be UID 0 and GID 0, respectively.  Any ranges which match that user and
+group in /etc/subuid and /etc/subgid are also mapped in as themselves with the
+help of the *newuidmap(1)* and *newgidmap(1)* helpers.
+
+podman unshare is useful for troubleshooting unprivileged operations and for
+manually clearing storage and other data related to images and containers.
+
+It is also useful if you want to use the `podman mount` command.  If an unprivileged users wants to mount and work with a container, then they need to execute
+podman unshare.  Executing `podman mount` fails for unprivileged users unless the user is running inside a `podman unshare` session.
+
+## EXAMPLE
+
+```
+$ podman unshare id
+uid=0(root) gid=0(root) groups=0(root),65534(nobody)
+
+$ podman unshare cat /proc/self/uid_map /proc/self/gid_map
+         0       1000          1
+         1      10000      65536
+         0       1000          1
+         1      10000      65536
+```
+
+
+## SEE ALSO
+podman(1), podman-mount(1), namespaces(7), newuidmap(1), newgidmap(1), user\_namespaces(7)
\ No newline at end of file
diff --git a/docs/podman.1.md b/docs/podman.1.md
index ef12cf1cc..ff942a3c4 100644
--- a/docs/podman.1.md
+++ b/docs/podman.1.md
@@ -128,58 +128,59 @@ the exit codes follow the `chroot` standard, see below:
 
 ## COMMANDS
 
-| Command                                   | Description                                                                    |
-| ----------------------------------------- | ------------------------------------------------------------------------------ |
-| [podman-attach(1)](podman-attach.1.md)    | Attach to a running container.                                                 |
-| [podman-build(1)](podman-build.1.md)      | Build a container image using a Dockerfile.                                    |
-| [podman-commit(1)](podman-commit.1.md)    | Create new image based on the changed container.                               |
-| [podman-container(1)](podman-container.1.md)    | Manage containers.                                                       |
-| [podman-cp(1)](podman-cp.1.md)            | Copy files/folders between a container and the local filesystem.               |
-| [podman-create(1)](podman-create.1.md)    | Create a new container.                                                        |
-| [podman-diff(1)](podman-diff.1.md)        | Inspect changes on a container or image's filesystem.                          |
-| [podman-events(1)](podman-events.1.md)    | Monitor Podman events                                                          |
-| [podman-exec(1)](podman-exec.1.md)        | Execute a command in a running container.                                      |
-| [podman-export(1)](podman-export.1.md)    | Export a container's filesystem contents as a tar archive.                     |
-| [podman-generate(1)](podman-generate.1.md)| Generate structured data based for a containers and pods.                      |
-| [podman-healthcheck(1)](podman-healthcheck.1.md)| Manage healthchecks for containers                                       |
-| [podman-history(1)](podman-history.1.md)  | Show the history of an image.                                                  |
-| [podman-image(1)](podman-image.1.md)      | Manage images.                                                                 |
-| [podman-images(1)](podman-images.1.md)    | List images in local storage.                                                  |
-| [podman-import(1)](podman-import.1.md)    | Import a tarball and save it as a filesystem image.                            |
-| [podman-info(1)](podman-info.1.md)        | Displays Podman related system information.                                    |
-| [podman-init(1)](podman-init.1.md)        | Initialize a container                                                         |
-| [podman-inspect(1)](podman-inspect.1.md)  | Display a container or image's configuration.                                  |
-| [podman-kill(1)](podman-kill.1.md)        | Kill the main process in one or more containers.                               |
-| [podman-load(1)](podman-load.1.md)        | Load an image from a container image archive into container storage.           |
-| [podman-login(1)](podman-login.1.md)      | Login to a container registry.                                                 |
-| [podman-logout(1)](podman-logout.1.md)    | Logout of a container registry.                                                |
-| [podman-logs(1)](podman-logs.1.md)        | Display the logs of a container.                                               |
-| [podman-mount(1)](podman-mount.1.md)      | Mount a working container's root filesystem.                                   |
-| [podman-pause(1)](podman-pause.1.md)      | Pause one or more containers.                                                  |
-| [podman-play(1)](podman-play.1.md)        | Play pods and containers based on a structured input file.                     |
-| [podman-pod(1)](podman-pod.1.md)          | Management tool for groups of containers, called pods.                         |
-| [podman-port(1)](podman-port.1.md)        | List port mappings for a container.                                            |
-| [podman-ps(1)](podman-ps.1.md)            | Prints out information about containers.                                       |
-| [podman-pull(1)](podman-pull.1.md)        | Pull an image from a registry.                                                 |
-| [podman-push(1)](podman-push.1.md)        | Push an image from local storage to elsewhere.                                 |
-| [podman-restart(1)](podman-restart.1.md)  | Restart one or more containers.                                                |
-| [podman-rm(1)](podman-rm.1.md)            | Remove one or more containers.                                                 |
-| [podman-rmi(1)](podman-rmi.1.md)          | Removes one or more locally stored images.                                     |
-| [podman-run(1)](podman-run.1.md)          | Run a command in a new container.                                              |
-| [podman-save(1)](podman-save.1.md)        | Save an image to a container archive.                                          |
-| [podman-search(1)](podman-search.1.md)    | Search a registry for an image.                                                |
-| [podman-start(1)](podman-start.1.md)      | Start one or more containers.                                                  |
-| [podman-stats(1)](podman-stats.1.md)      | Display a live stream of one or more container's resource usage statistics.    |
-| [podman-stop(1)](podman-stop.1.md)        | Stop one or more running containers.                                           |
-| [podman-system(1)](podman-system.1.md)    | Manage podman.                                                                 |
-| [podman-tag(1)](podman-tag.1.md)          | Add an additional name to a local image.                                       |
-| [podman-top(1)](podman-top.1.md)          | Display the running processes of a container.                                  |
-| [podman-umount(1)](podman-umount.1.md)    | Unmount a working container's root filesystem.                                 |
-| [podman-unpause(1)](podman-unpause.1.md)  | Unpause one or more containers.                                                |
-| [podman-version(1)](podman-varlink.1.md)  | Runs the varlink backend interface.                                            |
-| [podman-varlink(1)](podman-version.1.md)  | Display the Podman version information.                                        |
-| [podman-volume(1)](podman-volume.1.md)    | Manage Volumes.                                                                |
-| [podman-wait(1)](podman-wait.1.md)        | Wait on one or more containers to stop and print their exit codes.             |
+| Command                                          | Description                                                                 |
+| ------------------------------------------------ | --------------------------------------------------------------------------- |
+| [podman-attach(1)](podman-attach.1.md)           | Attach to a running container.                                              |
+| [podman-build(1)](podman-build.1.md)             | Build a container image using a Dockerfile.                                 |
+| [podman-commit(1)](podman-commit.1.md)           | Create new image based on the changed container.                            |
+| [podman-container(1)](podman-container.1.md)     | Manage containers.                                                          |
+| [podman-cp(1)](podman-cp.1.md)                   | Copy files/folders between a container and the local filesystem.            |
+| [podman-create(1)](podman-create.1.md)           | Create a new container.                                                     |
+| [podman-diff(1)](podman-diff.1.md)               | Inspect changes on a container or image's filesystem.                       |
+| [podman-events(1)](podman-events.1.md)           | Monitor Podman events                                                       |
+| [podman-exec(1)](podman-exec.1.md)               | Execute a command in a running container.                                   |
+| [podman-export(1)](podman-export.1.md)           | Export a container's filesystem contents as a tar archive.                  |
+| [podman-generate(1)](podman-generate.1.md)       | Generate structured data based for a containers and pods.                   |
+| [podman-healthcheck(1)](podman-healthcheck.1.md) | Manage healthchecks for containers                                          |
+| [podman-history(1)](podman-history.1.md)         | Show the history of an image.                                               |
+| [podman-image(1)](podman-image.1.md)             | Manage images.                                                              |
+| [podman-images(1)](podman-images.1.md)           | List images in local storage.                                               |
+| [podman-import(1)](podman-import.1.md)           | Import a tarball and save it as a filesystem image.                         |
+| [podman-info(1)](podman-info.1.md)               | Displays Podman related system information.                                 |
+| [podman-init(1)](podman-init.1.md)               | Initialize a container                                                      |
+| [podman-inspect(1)](podman-inspect.1.md)         | Display a container or image's configuration.                               |
+| [podman-kill(1)](podman-kill.1.md)               | Kill the main process in one or more containers.                            |
+| [podman-load(1)](podman-load.1.md)               | Load an image from a container image archive into container storage.        |
+| [podman-login(1)](podman-login.1.md)             | Login to a container registry.                                              |
+| [podman-logout(1)](podman-logout.1.md)           | Logout of a container registry.                                             |
+| [podman-logs(1)](podman-logs.1.md)               | Display the logs of a container.                                            |
+| [podman-mount(1)](podman-mount.1.md)             | Mount a working container's root filesystem.                                |
+| [podman-pause(1)](podman-pause.1.md)             | Pause one or more containers.                                               |
+| [podman-play(1)](podman-play.1.md)               | Play pods and containers based on a structured input file.                  |
+| [podman-pod(1)](podman-pod.1.md)                 | Management tool for groups of containers, called pods.                      |
+| [podman-port(1)](podman-port.1.md)               | List port mappings for a container.                                         |
+| [podman-ps(1)](podman-ps.1.md)                   | Prints out information about containers.                                    |
+| [podman-pull(1)](podman-pull.1.md)               | Pull an image from a registry.                                              |
+| [podman-push(1)](podman-push.1.md)               | Push an image from local storage to elsewhere.                              |
+| [podman-restart(1)](podman-restart.1.md)         | Restart one or more containers.                                             |
+| [podman-rm(1)](podman-rm.1.md)                   | Remove one or more containers.                                              |
+| [podman-rmi(1)](podman-rmi.1.md)                 | Removes one or more locally stored images.                                  |
+| [podman-run(1)](podman-run.1.md)                 | Run a command in a new container.                                           |
+| [podman-save(1)](podman-save.1.md)               | Save an image to a container archive.                                       |
+| [podman-search(1)](podman-search.1.md)           | Search a registry for an image.                                             |
+| [podman-start(1)](podman-start.1.md)             | Start one or more containers.                                               |
+| [podman-stats(1)](podman-stats.1.md)             | Display a live stream of one or more container's resource usage statistics. |
+| [podman-stop(1)](podman-stop.1.md)               | Stop one or more running containers.                                        |
+| [podman-system(1)](podman-system.1.md)           | Manage podman.                                                              |
+| [podman-tag(1)](podman-tag.1.md)                 | Add an additional name to a local image.                                    |
+| [podman-top(1)](podman-top.1.md)                 | Display the running processes of a container.                               |
+| [podman-umount(1)](podman-umount.1.md)           | Unmount a working container's root filesystem.                              |
+| [podman-unpause(1)](podman-unpause.1.md)         | Unpause one or more containers.                                             |
+| [podman-unshare(1)](podman-unshare.1.md)         | Run a command inside of a modified user namespace.                          |
+| [podman-version(1)](podman-varlink.1.md)         | Runs the varlink backend interface.                                         |
+| [podman-varlink(1)](podman-version.1.md)         | Display the Podman version information.                                     |
+| [podman-volume(1)](podman-volume.1.md)           | Manage Volumes.                                                             |
+| [podman-wait(1)](podman-wait.1.md)               | Wait on one or more containers to stop and print their exit codes.          |
 
 ## FILES