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)

Dockerfile

@@ -8,7 +8,10 @@ COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
 
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build \
+ARG TARGETOS=linux
+ARG TARGETARCH=amd64
+
+RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build \
     -ldflags="-s -w" \
     -o /build/vaultwarden-api \
     ./cmd/api