Docker containers used to support the attribute `additionaltags` which
was used to provide multiple tags for the same image. Since only tags
were supported this commit renames the attribute to `additionalnames`
and now supports tags and names witht he following syntax:
* '<name>:<tag>' -> adds a full docker image reference including name
and tag
* ':<tag>' -> adds an additional tag while reusing the former name
* '<name>' -> adds an additional name while reusing the former tag
Fixes#2045
Signed-off-by: David Cassany <dcassany@suse.com>
