compat API: allow enforcing short-names resolution to Docker Hub

The Docker-compatible REST API has historically behaved just as the rest
of Podman and Buildah (and the atomic Docker in older RHEL/Fedora) where
`containers-registries.conf` is centrally controlling which registries
a short name may resolve to during pull or local image lookups.  Please
refer to a blog for more details [1].

Docker, however, is only resolving short names to docker.io which has
been reported (see #12320) to break certain clients who rely on this
behavior.  In order to support this scenario, `containers.conf(5)`
received a new option to control whether Podman's compat API resolves
to docker.io only or behaves as before.

Most endpoints allow for directly normalizing parameters that represent
an image.  If set in containers.conf, Podman will then normalize the
references directly to docker.io.  The build endpoint is an outlier
since images are also referenced in Dockerfiles.  The Buildah API,
however, supports specifying a custom `types.SystemContext` in which
we can set a field that enforces short-name resolution to docker.io
in `c/image/pkg/shortnames`.

Notice that this a "hybrid" approach of doing the normalization directly
in the compat endpoints *and* in `pkg/shortnames` by passing a system
context.  Doing such a hybrid approach is neccessary since the compat
and the libpod endpoints share the same `libimage.Runtime` which makes
a global enforcement via the `libimage.Runtime.systemContext`
impossible.  Having two separate runtimes for the compat and the libpod
endpoints seems risky and not generally applicable to all endpoints.

[1] https://www.redhat.com/sysadmin/container-image-short-names

Fixes: #12320
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/test/apiv2/containers.conf b/test/apiv2/containers.conf
new file mode 100644
index 000000000..24762192f
--- /dev/null
+++ b/test/apiv2/containers.conf
@@ -0,0 +1,8 @@
+# This containers.conf file is used to test enforcing short-name resolution to
+# docker.io for Podman's *compat* API.  By default, the compat API defaults to
+# resolving to docker.io only.  The behavior can be altered by configuring the
+# containers.conf as done below in which case short names are subject to aliases,
+# "localhost/" and the unqualified-search registries.
+
+[engine]
+compat_api_enforce_docker_hub=false