tests/docker: add "fetch" sub-command

This simply wraps up fetching a build from the registry and tagging it
as the local build.

Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210512102051.12134-6-alex.bennee@linaro.org>

tests/docker/docker.py

@@ -515,6 +515,23 @@ def run(self, args, argv):
 
         return 0
 
+class FetchCommand(SubCommand):
+    """ Fetch a docker image from the registry. Args: <tag> <registry>"""
+    name = "fetch"
+
+    def args(self, parser):
+        parser.add_argument("tag",
+                            help="Local tag for image")
+        parser.add_argument("registry",
+                            help="Docker registry")
+
+    def run(self, args, argv):
+        dkr = Docker()
+        dkr.command(cmd="pull", quiet=args.quiet,
+                    argv=["%s/%s" % (args.registry, args.tag)])
+        dkr.command(cmd="tag", quiet=args.quiet,
+                    argv=["%s/%s" % (args.registry, args.tag), args.tag])
+
 
 class UpdateCommand(SubCommand):
     """ Update a docker image. Args: <tag> <actions>"""