CI: set proper docker image version for PRs against stable branches

This solution is copy-pasted from pmemkv (70cd9e4).
Without this change PRs created against any stable branch may end up failing,
because it uses 'latest' image, instead of proper stable (e.g. '1.13') image.

Author: lukaszstolarczuk

.github/workflows/gha.yml

@@ -48,9 +48,16 @@ jobs:
           echo "FORCE_IMAGE_ACTION=rebuild" >> $GITHUB_ENV
 
       - name: Set image version and force image action for stable branch
-        if: startsWith(github.ref, 'refs/heads/stable-')
+        # we want to set IMG_VER to e.g. '1.x' for stable branches and PRs against them
+        # for PRs we have to use 'base_ref' - this is the target branch (and we have to check that instead)
+        if:
+          startsWith(github.ref, 'refs/heads/stable-') || startsWith(github.base_ref, 'stable-')
+        # we now know we're on (or against) stable branches, so we just need to pick the version (e.g. the mentioned '1.x')
         run: |
-          echo "IMG_VER=$(echo ${GITHUB_REF#refs/heads/} | cut -d - -f 2)" >> $GITHUB_ENV
+          IMG_VER=$(echo ${GITHUB_BASE_REF} | cut -d - -f 2)
+          [ -z "${IMG_VER}" ] \
+              && echo "IMG_VER=$(echo ${GITHUB_REF#refs/heads/} | cut -d - -f 2)" >> $GITHUB_ENV \
+              || echo "IMG_VER=${IMG_VER}" >> $GITHUB_ENV
           echo "FORCE_IMAGE_ACTION=rebuild" >> $GITHUB_ENV
 
       - name: Clone the git repo