fix: add docker-setup action, runtime Dockerfile, and align release workflow

- Add .github/actions/docker-setup composite action (from rivet)
- Add docker/runtime/Dockerfile for Docker image builds
- Update release.yaml to match rivet patterns:
  - Use corepack enable instead of pnpm/action-setup
  - Add reuse_engine_version input
  - Add Docker job with Depot runners
  - Use --no-frozen-lockfile for pnpm install
  - Add id-token permission for setup job

.github/actions/docker-setup/action.yaml

@@ -0,0 +1,31 @@
+name: 'Docker Setup'
+description: 'Set up Docker Buildx and log in to Docker Hub'
+inputs:
+  docker_username:
+    description: 'Docker Hub username'
+    required: true
+  docker_password:
+    description: 'Docker Hub password'
+    required: true
+  github_token:
+    description: 'GitHub token'
+    required: true
+runs:
+  using: 'composite'
+  steps:
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+
+    - name: Log in to Docker Hub
+      uses: docker/login-action@v3
+      with:
+        username: ${{ inputs.docker_username }}
+        password: ${{ inputs.docker_password }}
+
+    # This will be used as a secret to authenticate with Git repo pulls
+    - name: Create .netrc file
+      run: |
+        echo "machine github.com" > ${{ runner.temp }}/netrc
+        echo "login x-access-token" >> ${{ runner.temp }}/netrc
+        echo "password ${{ inputs.github_token }}" >> ${{ runner.temp }}/netrc
+      shell: bash