Be helpful to CI/CD job maintainers

It's often hard to debug/maintain/improve a job when you can't observe
any of the (many!) auto-generated CI env. vars.  Print them all out on
every job.

Signed-off-by: Chris Evich <chris_gitlab@icuc.me>

.gitlab-ci.yml

@@ -17,10 +17,14 @@ build:
   script:
     - 'mkdir -p /kaniko/.docker'
     - 'echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json'
+    - |
+        echo "Select CI env. vars.:";
+        printenv | egrep '^CI_' | sort
+    # N/B: There could be more than one merge-request open with this branch's HEAD
     - |
         IMAGE_TAG="${CI_COMMIT_TAG:+${CI_COMMIT_TAG}}${CI_COMMIT_BRANCH:+${CI_COMMIT_BRANCH/main/latest}}";
         if [[ -n "$CI_OPEN_MERGE_REQUESTS" ]]; then
-            IMAGE_TAG="mr${CI_MERGE_REQUEST_ID}";
+            IMAGE_TAG=mr$(echo "${CI_OPEN_MERGE_REQUESTS}" | cut -d, -f -1 | cut -d\! -f 2);
         fi
         echo "Building/Pushing to: ${CI_REGISTRY_IMAGE}:${IMAGE_TAG}";
     - |