add in comments for knarly substring line

.github/workflows/docker.yml

@@ -38,7 +38,11 @@ jobs:
       -
         name: Identify GitHub tag name
         shell: bash
-        run: echo "##[set-output name=tag;]$(echo ${GITHUB_REF#refs/tags/})"
+        # get tag name from runners environment 'GITHUB_REF' and then use bash substring
+        # to strip out '+' symbol - required due to gcr not supporting this as a tag 
+        # name (docker hub does support it).
+        # note if push is NOT triggered by tag then 'GITHUB_REF' will be the branch name.
+        run: echo "##[set-output name=tag;]$(tag_name=${GITHUB_REF#refs/tags/} && echo "${tag_name//+/-}")"
         id: identify_tag
       -
         name: Build and Push to GCR and Docker Hub