ci: Make Docker workflows run on tag pushes

Since we moved the Docker workflows to Github (after v1.10), they have
not been running on tags, so there are no tagged docker images for
v1.11, v1.12 and v1.13.

This is (hopefully) because we're not explicitly asking for the workflow
to be run on tag pushes.

This patch (hopefully) fixes that by adding an explicit section in the
config to make it run on tag pushes.

Thanks to Christoph Mewes (xrstf@github) for reporting this in
#51.

.github/workflows/docker.yml

@@ -3,6 +3,7 @@ name: "docker"
 on:
   push:
     branches: [ "master", "next" ]
+    tags: [ "v*" ]
   pull_request:
     # The branches below must be a subset of the branches above
     branches: [ "master", "next" ]