Publish docker images for minor versions

[avdata99]

It will be nice to have docker images for CKAN 2.9.6, 2.8.11 and next minor releases

This PR also includes a template for PRs. I tried to use another approach to avoid forgetting to update the version in the github action files, but I didn't like it

[amercader]

I like the idea, but not the implementation. It will be too easy for us to forget to edit the version in two different places, and the PR checklist will not apply to most PRs. We should keep the GIT_BRANCH env var as the single source of truth for the version being built. In the workflow definition we can extract the version in a first step and use it in the following ones:

diff --git a/.github/workflows/publish-docker.yml b/.github/workflows/publish-docker.yml
index 3123abf..07e4f30 100644
--- a/.github/workflows/publish-docker.yml
+++ b/.github/workflows/publish-docker.yml
@@ -17,45 +17,49 @@ jobs:
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Set CKAN Versions
+        run: |
+          echo CKAN_VERSION_2.8=$(grep 'ENV GIT_BRANCH' ckan-base/2.8/Dockerfile | cut -d '-' -f 2) >> $GITHUB_ENV
+          echo CKAN_VERSION_2.9=$(grep 'ENV GIT_BRANCH' ckan-base/2.9/Dockerfile | cut -d '-' -f 2) >> $GITHUB_ENV
       - name: Build ckan-base 2.8
         uses: docker/build-push-action@v2
         with:
           context: ckan-base
           file: ckan-base/2.8/Dockerfile
           push: true
-          tags: openknowledge/ckan-base:2.8
+          tags: openknowledge/ckan-base:2.8,openknowledge/ckan-base:${{ env.CKAN_VERSION_2.8 }}
       - name: Build ckan-base 2.9
         uses: docker/build-push-action@v2
         with:
           context: ckan-base
           file: ckan-base/2.9/Dockerfile
           push: true
-          tags: openknowledge/ckan-base:2.9
+          tags: openknowledge/ckan-base:2.9,openknowledge/ckan-base:${{ env.CKAN_VERSION_2.9 }}
       - name: Build ckan-base 2.9-py2
         uses: docker/build-push-action@v2
         with:
           context: ckan-base
           file: ckan-base/2.9/Dockerfile.py2
           push: true
-          tags: openknowledge/ckan-base:2.9-py2
+          tags: openknowledge/ckan-base:2.9-py2,openknowledge/ckan-base:${{ env.CKAN_VERSION_2.9 }}-py2
       - name: Build ckan-dev 2.8
         uses: docker/build-push-action@v2
         with:
           context: ckan-dev
           file: ckan-dev/2.8/Dockerfile
           push: true
-          tags: openknowledge/ckan-dev:2.8
+          tags: openknowledge/ckan-dev:2.8,openknowledge/ckan-dev:${{ env.CKAN_VERSION_2.8 }}
       - name: Build ckan-dev 2.9
         uses: docker/build-push-action@v2
         with:
           context: ckan-dev
           file: ckan-dev/2.9/Dockerfile
           push: true
-          tags: openknowledge/ckan-dev:2.9
+          tags: openknowledge/ckan-dev:2.9,openknowledge/ckan-dev:${{ env.CKAN_VERSION_2.9 }}
       - name: Build ckan-dev 2.9-py2
         uses: docker/build-push-action@v2
         with:
           context: ckan-dev
           file: ckan-dev/2.9/Dockerfile.py2
           push: true
-          tags: openknowledge/ckan-dev:2.9-py2
+          tags: openknowledge/ckan-dev:2.9-py2,openknowledge/ckan-dev:${{ env.CKAN_VERSION_2.9 }}-py2

[amercader]

Looks good @avdata99, let's give it a go!