feat: build-on-node — git checkouts, remote image builds, image transport (#60)

Git-sourced deploys never materialized a checkout: sourcePath was
build_path resolved against the server cwd, so builds consumed the
backend's own directory. Remote placement could only run
registry-pulled images.

- internal/source: per-deployment checkout (init/fetch/checkout) at
  branch or commit; GIT_ASKPASS keeps tokens out of argv, .git/config,
  and the build context; dumb-transport and SHA-refusal fallbacks;
  build_path validated inside the checkout.
- BuildManager.PackageContext: produces the same tar context Build
  ships to the daemon — generated Dockerfile at tar root, .git
  excluded. Fixes nixpacks passing the Dockerfile path as content.
- Agent artifact endpoint: GET /api/agents/artifacts/:id streams
  build contexts and docker-save tarballs behind agent-token auth;
  hex-validated ids, per-deployment cleanup, boot-time purge.
- Agent commands: build_image (docker build -, --build-arg values via
  process env, not argv) and load_image (docker load for containr-*
  rollback tags no registry serves).
- Deployment flow: remote git builds package context → artifact →
  node docker-builds the deployment tag; remote rollbacks of local
  tags export via SaveImage → artifact → node docker-loads.

Verified end-to-end: agent-pinned git service built on the node
(build_image → create_container → deployed) and rollback shipped a
docker-save tarball (load_image → deployed). Artifact auth returns
401/404 correctly; workspaces and blobs are removed after deploy.

Authored By: TDvorak <[email protected]>

app/backend/internal/build/nixpacks.go

@@ -476,17 +476,22 @@ func (n *NixpacksBuilder) Build(ctx context.Context, req *types.BuildRequest) (*
 	}
 
 	// Create temporary Dockerfile
-	dockerfile, err := n.generateDockerfile(plan)
+	dockerfilePath, err := n.generateDockerfile(plan)
 	if err != nil {
 		return nil, fmt.Errorf("failed to generate Dockerfile: %w", err)
 	}
-	defer os.Remove(dockerfile)
+	defer os.Remove(dockerfilePath)
 
 	// Build Docker image
 	imageName := fmt.Sprintf("%s:%s", req.ImageName, req.ImageTag)
 
-	// Create build context tar
-	buildCtx, err := createBuildContext(req.SourcePath, dockerfile)
+	// Create build context tar — createBuildContext takes the Dockerfile
+	// contents, not its path.
+	dockerfileData, err := os.ReadFile(dockerfilePath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read generated Dockerfile: %w", err)
+	}
+	buildCtx, err := createBuildContext(req.SourcePath, string(dockerfileData))
 	if err != nil {
 		return nil, fmt.Errorf("failed to create build context: %w", err)
 	}