chore: multi-arch Docker build (amd64, arm64, armv7) (#4)

* chore: multi-arch Docker build (amd64, arm64, armv7)

- Remove hardcoded GOARCH=amd64 from Dockerfile, use TARGETARCH ARG
- Add QEMU + buildx to CI for cross-compilation
- Build for linux/amd64, linux/arm64, linux/arm/v7

* chore: update action versions, add dependabot

- actions/checkout v4 → v6
- actions/setup-go v5 → v6
- Add dependabot.yml (monthly checks for Go, Actions, Docker)

.github/workflows/ci.yml

@@ -10,9 +10,9 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@v6
         with:
           go-version-file: go.mod
 
@@ -30,7 +30,11 @@ jobs:
       contents: read
       packages: write
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
+
+      - uses: docker/setup-qemu-action@v3
+
+      - uses: docker/setup-buildx-action@v3
 
       - uses: docker/login-action@v3
         with:
@@ -42,4 +46,5 @@ jobs:
         with:
           context: .
           push: true
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
           tags: ghcr.io/turbootzz/vaultwarden-api:latest