Fix image tagging (again)

This CI environment is using a busybox `sh` so doesn't support all the
advanced features of bash.  Reimplement IMAGE_TAG processing so it
functions as intended.

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

.gitlab-ci.yml

@@ -22,9 +22,13 @@ build:
         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 "$CI_OPEN_MERGE_REQUESTS" ]]; then
+        IMAGE_TAG="${CI_COMMIT_BRANCH}";
+        if [[ -n "$CI_COMMIT_TAG" ]]; then
+            IMAGE_TAG="${CI_COMMIT_TAG}";
+        elif [[ -n "$CI_OPEN_MERGE_REQUESTS" ]]; then
             IMAGE_TAG=mr$(echo "${CI_OPEN_MERGE_REQUESTS}" | cut -d, -f -1 | cut -d\! -f 2);
+        elif [[ "$CI_COMMIT_BRANCH" == "main" ]]; then
+            IMAGE_TAG="latest";
         fi
         echo "Building/Pushing to: ${CI_REGISTRY_IMAGE}:${IMAGE_TAG}";
     - |