fix(build): clear a killed boot test's container and volume first

When the worker is restarted mid-build (watchtower updating its image), the
boot test's container and volume are left behind, and the next boot test of
that game deploys under the same name. Remove them before each boot test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/gbrgames/build.go

@@ -236,6 +236,10 @@ func bootCheck(gamesDir, id, agentImage string, wantOutside bool) (problems, ver
 	if err != nil {
 		return nil, nil, err
 	}
+	// A boot test that was killed (worker restarted mid-build) leaves its container and volume behind,
+	// and the next deploy under the same name would trip over them
+	runCmd("docker", "rm", "-f", "boottest-"+id)
+	runCmd("docker", "volume", "rm", "-f", "boottest-"+id+"-data")
 	payloadCmd := exec.Command(self, "boottest-payload", "-games", gamesDir, id)
 	var stderr bytes.Buffer
 	payloadCmd.Stderr = &stderr