feat: add target input for multi-stage Docker builds

Allows callers to specify a Docker build target stage (e.g. target: production)
for multi-stage Dockerfiles. Empty default preserves backward compatibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build-push.yaml

@@ -29,6 +29,11 @@ on:
         required: false
         type: string
         default: ""
+      target:
+        description: "Docker build target stage (for multi-stage builds)"
+        required: false
+        type: string
+        default: ""
       runner:
         description: "Runner label to use (e.g. ubuntu-latest, self-hosted-arm64). ARM64 builds use QEMU emulation on amd64 runners by default — set this to a native ARM64 runner for faster Rust/heavy builds."
         required: false
@@ -103,6 +108,7 @@ jobs:
         with:
           context: ${{ inputs.context }}
           file: ${{ inputs.context }}/${{ inputs.dockerfile }}
+          target: ${{ inputs.target || '' }}
           platforms: ${{ inputs.platforms }}
           push: true
           tags: ${{ steps.meta.outputs.tags }}