Optimize CI build performance - build amd64 only for main branch

The multi-architecture builds (amd64 + arm64) were taking 15-20+ minutes
due to QEMU emulation for ARM64 cross-compilation. This is too slow for
regular development iteration.

Changes:
- Main branch builds: amd64 only (much faster, ~3-5 minutes)
- Tagged releases: Still build multi-arch (amd64 + arm64)
- Added Docker layer caching to speed up subsequent builds
- Cache stored in registry for persistence across CI runs

The 'latest' tag will be amd64-only from main branch. Users needing
ARM64 should use a specific version tag (e.g., v0.1.0).

.gitlab-ci.yml

@@ -56,7 +56,7 @@ build:test:
     - main
     - tags
 
-# Build and push for main branch (multi-architecture)
+# Build and push for main branch (amd64 only for speed)
 build:main:
   stage: build
   image: docker:24-dind
@@ -64,13 +64,14 @@ build:main:
     - docker:24-dind
   before_script:
     - echo "$DOCKERHUB_TOKEN" | docker login -u "$DOCKERHUB_USERNAME" --password-stdin
-    # Enable buildx for multi-arch
-    - docker buildx create --driver docker-container --name multiarch-builder --use || docker buildx use multiarch-builder
+    - docker buildx create --driver docker-container --name builder --use || docker buildx use builder
     - docker buildx inspect --bootstrap
   script:
     - |
       docker buildx build \
-        --platform linux/amd64,linux/arm64 \
+        --platform linux/amd64 \
+        --cache-from type=registry,ref=$REGISTRY/$IMAGE_NAME:buildcache \
+        --cache-to type=registry,ref=$REGISTRY/$IMAGE_NAME:buildcache,mode=max \
         --tag $REGISTRY/$IMAGE_NAME:latest \
         --tag $REGISTRY/$IMAGE_NAME:main-$CI_COMMIT_SHORT_SHA \
         --push \
@@ -82,7 +83,8 @@ build:main:
   only:
     - main
 
-# Release build for tags (multi-architecture)
+# Release build for tags (multi-architecture with caching)
+# Note: Multi-arch builds are slow (~15-20 min) due to QEMU emulation for ARM64
 release:multiarch:
   stage: release
   image: docker:24-dind
@@ -90,7 +92,6 @@ release:multiarch:
     - docker:24-dind
   before_script:
     - echo "$DOCKERHUB_TOKEN" | docker login -u "$DOCKERHUB_USERNAME" --password-stdin
-    # Enable buildx for multi-arch
     - docker buildx create --driver docker-container --name multiarch-builder --use || docker buildx use multiarch-builder
     - docker buildx inspect --bootstrap
   script:
@@ -99,6 +100,8 @@ release:multiarch:
 
       docker buildx build \
         --platform linux/amd64,linux/arm64 \
+        --cache-from type=registry,ref=$REGISTRY/$IMAGE_NAME:buildcache \
+        --cache-to type=registry,ref=$REGISTRY/$IMAGE_NAME:buildcache,mode=max \
         --tag $REGISTRY/$IMAGE_NAME:$VERSION \
         --tag $REGISTRY/$IMAGE_NAME:$CI_COMMIT_TAG \
         --tag $REGISTRY/$IMAGE_NAME:latest \