ci: split build and test into separate jobs, upgrade actions to Node 24

Previously each of the 21 test matrix cells built its own Docker image,
totalling 21 redundant builds per CI run.

New structure:
- build_images: 4 jobs (one per PG version), builds the test image and
  uploads it as a zstd-compressed artifact (retention: 1 day).
- run_tests: 21 jobs (same matrix as before), downloads the pre-built
  image artifact and runs tests against it via make run-test-prebuilt.

This cuts Docker builds from 21 to 4 per CI run.

Also upgrades all actions/checkout, actions/upload-artifact, and
actions/download-artifact from Node 20 to Node 24 (v7/v7/v8) to
silence the runner deprecation warning.

New Makefile targets:
  save-test-image   docker save | zstd → .tar.zst archive
  load-test-image   zstd decompress | docker load
  run-test-prebuilt like run-test but without the build prerequisite

Author: dimitri

.github/workflows/run-tests.yml

@@ -17,7 +17,7 @@ jobs:
     container: citus/stylechecker:no-py
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4.2.2
+        uses: actions/checkout@v7.0.0
 
       - name: Set safe directory for git
         run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
@@ -28,9 +28,38 @@ jobs:
       - name: Check banned functions
         run: ci/banned.h.sh
 
+  build_images:
+    name: Build test image (PG${{ matrix.PGVERSION }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        PGVERSION:
+          - 14
+          - 15
+          - 16
+          - 17
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v7.0.0
+
+      - name: Build Docker test image
+        run: PGVERSION=${{ matrix.PGVERSION }} make build-test-image
+
+      - name: Save Docker image to archive
+        run: PGVERSION=${{ matrix.PGVERSION }} make save-test-image
+
+      - name: Upload image archive
+        uses: actions/upload-artifact@v7.0.1
+        with:
+          name: pg-test-image-${{ matrix.PGVERSION }}
+          path: pg_auto_failover_test-pg${{ matrix.PGVERSION }}.tar.zst
+          retention-days: 1
+
   run_tests:
-    name: Run test
+    name: Run test (${{ matrix.PGVERSION }}, ${{ matrix.TEST }})
     runs-on: ubuntu-latest
+    needs: build_images
     strategy:
       fail-fast: false
       matrix:
@@ -50,17 +79,22 @@ jobs:
             TEST: tablespaces
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4.2.2
+        uses: actions/checkout@v7.0.0
+
+      - name: Download image archive
+        uses: actions/download-artifact@v8.0.1
+        with:
+          name: pg-test-image-${{ matrix.PGVERSION }}
+
+      - name: Load Docker test image
+        run: PGVERSION=${{ matrix.PGVERSION }} make load-test-image
 
       - name: Set environment variables
         run: |
           echo "PGVERSION=${{ matrix.PGVERSION }}" >> $GITHUB_ENV
           echo "TEST=${{ matrix.TEST }}" >> $GITHUB_ENV
           echo "TRAVIS_BUILD_DIR=$(pwd)" >> $GITHUB_ENV
 
-      - name: Build Docker Test Image
-        run: make build-test-image
-
       - name: Run Test
         timeout-minutes: 15
-        run: make ci-test
+        run: make run-test-prebuilt

Makefile

@@ -350,6 +350,31 @@ run-test: build-test-pg$(PGVERSION)
 		make -C /usr/src/pg_auto_failover test	\
 		PGVERSION=$(PGVERSION) TEST='${TEST}'
 
+# make run-test-prebuilt; like run-test but skips the build step.
+# Used in CI after images have been built and loaded by a prior job.
+.PHONY: run-test-prebuilt
+run-test-prebuilt:
+	docker run					                \
+		--name $(TEST_CONTAINER_NAME)		    \
+		$(DOCKER_RUN_OPTS)			            \
+		$(TEST_CONTAINER_NAME):pg$(PGVERSION)   \
+		make -C /usr/src/pg_auto_failover test	\
+		PGVERSION=$(PGVERSION) TEST='${TEST}'
+
+# make save-test-image; compresses the test image to a .tar.zst archive.
+# Used in CI to pass the built image to downstream test jobs as an artifact.
+.PHONY: save-test-image
+save-test-image:
+	docker save $(TEST_CONTAINER_NAME):pg$(PGVERSION) \
+	  | zstd -T0 -3 > $(TEST_CONTAINER_NAME)-pg$(PGVERSION).tar.zst
+
+# make load-test-image; decompresses and loads a .tar.zst image archive.
+# Used in CI test jobs that download the image from a prior build job.
+.PHONY: load-test-image
+load-test-image:
+	zstd -d --stdout $(TEST_CONTAINER_NAME)-pg$(PGVERSION).tar.zst \
+	  | docker load
+
 #
 # BE INTERACTIVE
 #