ci: introduce a workflow for populating Maven cache that can be shared amongst workflows

Part of #1656

[skip ci]

Author: php-coder

.github/dependabot.yml

@@ -38,6 +38,7 @@ updates:
     directory: "/"
     # https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#allow
     allow:
+      - dependency-name: "actions/cache"
       - dependency-name: "actions/checkout"
       - dependency-name: "actions/setup-java"
       - dependency-name: "actions/upload-artifact"

.github/workflows/populate-maven-cache.yml

@@ -0,0 +1,52 @@
+name: Populates a cache for Maven
+
+on:
+  push:
+    # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpull_requestpull_request_targetbranchesbranches-ignore
+    branches:
+      - master
+    # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore
+    paths:
+      - pom.xml
+      - .github/workflows/populate-maven-cache.yml
+  # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onworkflow_dispatch
+  workflow_dispatch:
+
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions
+permissions:
+  contents: read # for "git clone"
+
+defaults:
+  # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#defaultsrun
+  run:
+    # Enable fail-fast behavior using set -eo pipefail
+    # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference
+    shell: bash
+
+jobs:
+  populate-maven-cache:
+    name: Populate Maven cache
+    # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idruns-on
+    runs-on: ubuntu-20.04
+    steps:
+
+      - name: Clone source code
+        uses: actions/[email protected] # https://github.com/actions/checkout
+        with:
+          # Whether to configure the token or SSH key with the local git config. Default: true
+          persist-credentials: false
+
+      - name: Install JDK
+        uses: actions/[email protected] # https://github.com/actions/setup-java
+        with:
+          distribution: 'adopt' # https://github.com/actions/setup-java#supported-distributions
+          java-version: '8'     # https://github.com/actions/setup-java#supported-version-syntax
+
+      - name: Restore existing cache copy
+        uses: actions/[email protected] # https://github.com/actions/cache
+        with:
+          key: maven-repository-${{ hashFiles('pom.xml') }}
+          path: ~/.m2/repository
+
+      - name: Download all dependencies
+        run: mvn dependency:resolve dependency:resolve-plugins