fix bug remote-podman images --digests

Add `digest` field to Image struct and update API.md
Show image digests when using --digests in podman-remote.

```
$ PODMAN_VARLINK_ADDRESS="tcp:127.0.0.1:1234" sudo bin/podman-remote images --digests
REPOSITORY                  TAG      DIGEST                                                                    IMAGE ID       CREATED       SIZE
docker.io/library/alpine    latest   sha256:d05ecd4520cab5d9e5d877595fb0532aadcd6c90f4bbc837bc11679f704c4c82   5cb3aa00f899   2 weeks ago   5.79 MB
docker.io/library/busybox   latest   sha256:4415a904b1aca178c2450fd54928ab362825e863c0ad5452fd020e92f7a6a47e   d8233ab899d4   5 weeks ago   1.42 MB
```

Signed-off-by: Qi Wang <qiwan@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/API.md b/API.md
index f7f5525bf..3a095f0ba 100755
--- a/API.md
+++ b/API.md
@@ -1472,6 +1472,8 @@ gid_map [IDMap](#IDMap)
 
 id [string](https://godoc.org/builtin#string)
 
+digest [string](https://godoc.org/builtin#string)
+
 parentId [string](https://godoc.org/builtin#string)
 
 repoTags [[]string](#[]string)