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/dependabot.yml

@@ -0,0 +1,41 @@
+version: 2
+updates:
+  # Go modules
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "dependencies"
+    commit-message:
+      prefix: "chore(deps)"
+    groups:
+      go-dependencies:
+        patterns:
+          - "*"
+
+  # GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "dependencies"
+      - "ci"
+    commit-message:
+      prefix: "chore(ci)"
+    groups:
+      actions-dependencies:
+        patterns:
+          - "*"
+
+  # Docker
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "dependencies"
+      - "docker"
+    commit-message:
+      prefix: "chore(docker)"