feat(maven): enable caching for Maven dependencies

Configure Maven local repository to use /var/cache/melange/m2repository,
which is persisted when a cache-dir is provided. This allows Maven builds
to reuse downloaded dependencies across builds which is very useful
because some java projects have a lot of deps to
download (eg. apicurio-registry needs more than 1 GB deps).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: toabctl

pkg/build/pipelines/maven/configure-mirror.yaml

@@ -4,10 +4,19 @@ needs:
     - busybox
 pipeline:
   - runs: |
+      # Use melange's build cache for downloaded dependencies if mounted from host
+      MAVEN_LOCAL_REPO="/var/cache/melange/m2repository"
+      LOCAL_REPO_CONFIG=""
+      if [ -d "/var/cache/melange" ]; then
+        mkdir -p "$MAVEN_LOCAL_REPO"
+        LOCAL_REPO_CONFIG="<localRepository>${MAVEN_LOCAL_REPO}</localRepository>"
+      fi
+
       # Maven checks $USER/.m2, we set $HOME to /home/build but it hardcodes $USER somehow
       mkdir -p /root/.m2
       cat > /root/.m2/settings.xml <<EOF
         <settings>
+          ${LOCAL_REPO_CONFIG}
           <mirrors>
             <mirror>
               <id>google-maven-central</id>

pkg/build/pipelines/maven/pombump.yaml

@@ -33,6 +33,18 @@ inputs:
 
 pipeline:
   - runs: |
+      # Use melange's build cache for downloaded dependencies if mounted from host
+      MAVEN_LOCAL_REPO="/var/cache/melange/m2repository"
+      if [ -d "/var/cache/melange" ]; then
+        mkdir -p "$MAVEN_LOCAL_REPO"
+        mkdir -p /root/.m2
+        cat > /root/.m2/settings.xml <<EOF
+      <settings>
+        <localRepository>${MAVEN_LOCAL_REPO}</localRepository>
+      </settings>
+      EOF
+      fi
+
       PATCH_FILE_FLAG=""
       PROPERTIES_FILE_FLAG=""
       DEPENDENCIES_FLAG=""