DRY up cache loading in CI workflow

This adds a composite action to run the two cache-loading steps that are
reused across three of the jobs in the continuous integration workflow.
The second of these steps is a little long and detailed, and differs
slightly but meaningfully from a similar step in the build cache job, so
it might be useful to DRY this up. This also allows us to see the meat
of the post-cache jobs a little easier in the continuous integration
workflow

The `actions/checkout@v4` step is needed in each job in order to load
our action (and presumably also the external ones used in the composite
action)

It would be quite nice to use a YAML anchor or alias to do this kind of
reuse, but these are currently unsupported in GitHub Actions. They might
be on the way soon, so watch this space: actions/runner#1182

.github/actions/continuous-integration/load-cache/action.yml

@@ -0,0 +1,20 @@
+name: Load cache
+description: Loads the cached image in a post-cache continuous integration step
+runs:
+  using: "composite"
+  steps:
+    -
+      name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+    -
+      name: Load cache image
+      uses: docker/build-push-action@v6
+      with:
+        context: .
+        file: ./Dockerfile
+        build-args: |
+          RAILS_ENV=test
+        push: false
+        load: true
+        tags: app_test:latest
+        cache-from: type=gha

.github/workflows/continuous-integration.yml

@@ -41,20 +41,8 @@ jobs:
         name: Checkout
         uses: actions/checkout@v4
       -
-        name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-      -
-        name: Build and cache
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          file: ./Dockerfile
-          build-args: |
-            RAILS_ENV=test
-          push: false
-          load: true
-          tags: app_test:latest
-          cache-from: type=gha
+        name: Load cache
+        uses: ./.github/actions/continuous-integration/load-cache
       -
         name: Run linters and formatters
         run: |
@@ -72,20 +60,8 @@ jobs:
         name: Checkout
         uses: actions/checkout@v4
       -
-        name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-      -
-        name: Build and cache
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          file: ./Dockerfile
-          build-args: |
-            RAILS_ENV=test
-          push: false
-          load: true
-          tags: app_test:latest
-          cache-from: type=gha
+        name: Load cache
+        uses: ./.github/actions/continuous-integration/load-cache
       -
         name: Run Brakeman
         run: |
@@ -100,20 +76,8 @@ jobs:
         name: Checkout
         uses: actions/checkout@v4
       -
-        name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-      -
-        name: Build and cache
-        uses: docker/build-push-action@v6
-        with:
-          context: .
-          file: ./Dockerfile
-          build-args: |
-            RAILS_ENV=test
-          push: false
-          load: true
-          tags: app_test:latest
-          cache-from: type=gha
+        name: Load cache
+        uses: ./.github/actions/continuous-integration/load-cache
       -
         name: Run RSpec and Simplecov
         run: |