ci: error if FRONTEND_VERSION not found (#2050)

Re #2042 (comment)

Adding `-e` isn't enough to cause a helpful failure when `FRONTEND_VERSION` is missing from `docker-compose.yml` because:

1. it doesn't fail, despite `bash -e`, because the `jq` command runs inside the `echo` call, which succeeds, and
2. if the intention is for the failure to be easy to understand, `jq -e` won't help on its own

Author: alxndrsn

.github/workflows/main.yml

@@ -96,13 +96,7 @@ jobs:
 
     - run: ./test/check-docker-context.sh --min-size 5000 --max-size 10000 --min-count 600 --max-count 700
 
-    - name: Extract FRONTEND_VERSION
-      run: |
-           touch .env
-           echo "FRONTEND_VERSION=$(
-             docker compose config --format json | jq -r .services.nginx.build.args.FRONTEND_VERSION
-           )" >> "$GITHUB_ENV"
-
+    - run: ./files/ci/extract-frontend-version.sh
     - run: FRONTEND_BUILD_MODE=fetch ./test/test-images.sh
 
     # Check out the current frontend version referenced by docker-compose, as it should build OK.
@@ -162,12 +156,7 @@ jobs:
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v4
 
-    - name: Extract FRONTEND_VERSION
-      run: |
-           touch .env
-           echo "FRONTEND_VERSION=$(
-             docker compose config --format json | jq -r .services.nginx.build.args.FRONTEND_VERSION
-           )" >> "$GITHUB_ENV"
+    - run: ./files/ci/extract-frontend-version.sh
 
     - name: Build and push ${{ matrix.image }} Docker image
       uses: docker/build-push-action@v7

files/ci/extract-frontend-version.sh

@@ -0,0 +1,21 @@
+#!/bin/bash -eu
+set -o pipefail
+shopt -s inherit_errexit
+
+log() { echo >&2 "[$(basename "$0")] $*"; }
+
+log "Ensuring .env exists for docker compose..."
+touch .env
+
+log "Reading FRONTEND_VERSION from docker-compose.yml..."
+if ! frontendVersion="$(docker compose config --format json | jq -er .services.nginx.build.args.FRONTEND_VERSION)"; then
+  log "!!!"
+  log "!!! Failed to read FRONTEND_VERSION from docker-compose.yml (got: '$frontendVersion')."
+  log "!!!"
+  exit 1
+fi
+
+log "Writing FRONTEND_VERSION to GITHUB_ENV file ($GITHUB_ENV)..."
+echo "FRONTEND_VERSION=$frontendVersion" >> "$GITHUB_ENV"
+
+log "Completed OK."