Fixup from Franco's feedback

Author: noonio

.github/workflows/docker.yaml

@@ -59,8 +59,13 @@ jobs:
 
         # Compute tags. We want the following behaviour:
         #
-        # 1. Building on tag:      image tagged `tag`
-        # 2. Building on release:  image tagged `latest`
+        # 1. Building on tag:
+        #
+        #    a. Image tagged `<tag>`
+        #
+        #    b. If the tag is the "latest" one for the present major release,
+        #    then also tag it as "latest"
+        #
         # 3. Building on master:   image tagged `unstable`.
 
         # Let's default to 'unstable'.
@@ -75,13 +80,8 @@ jobs:
           IMAGE_LABEL=${{github.ref_name}} && \
           VERSION=${{github.ref_name}}
 
-        # If we're running on the release branch, tag it as `latest`,
-        # and leave the version as the git commit.
-        [[ ${GITHUB_REF_NAME} = "release" ]] && \
-          IMAGE_LABEL=latest
-
         # If we're running on master, tag it as "unstable" and
-        # and again leave the version as the git commit.
+        # and leave the version as the git commit.
         [[ ${GITHUB_REF_NAME} = "master" ]] && \
           IMAGE_LABEL=unstable
 
@@ -95,6 +95,27 @@ jobs:
           --label org.opencontainers.image.version=${VERSION} \
           --tag ${IMAGE_NAME}:${IMAGE_LABEL} -
 
+        # Tag the build as 'latest' if it's the latest tag.
+        if [[ ${BUILDING_TAG} = true ]]; then
+          # Build a sorted list of tags . We have to grep-exclude some tags
+          # that we don't want to include; i.e. ones that don't point to our
+          # actual releases.
+          git tag -l | grep -P '^\d+.\d+.\d+$' >tags.txt
+
+          # Get the largest one (sort -V sorts as versions).
+          sort -V tags.txt | tail -n1 >latest.txt
+
+          # Compare it to mine; and if it's the same, then I am the latest, so
+          # we can tag it as such.
+          echo ${{ github.ref_name }}>mine.txt
+
+          # Note: This will also print the diff if it fails, which is useful
+          # for debugging.
+          if (diff mine.txt latest.txt); then
+            docker tag ${IMAGE_NAME}:${IMAGE_LABEL} ${IMAGE_NAME}:latest
+          fi
+        fi
+
         # Also tag with ref name when manually dispatched
         [[ ${{github.event_name == 'workflow_dispatch'}} = true ]] && \
           docker tag ${IMAGE_NAME}:${IMAGE_LABEL} ${IMAGE_NAME}:workflow_dispatch-${{github.event.inputs.ref_name}}