fix(ci): add Dockerfile.prebuilt for multi-arch API builds

Was gitignored, causing CI to fail with "no such file" error.
Uses TARGETARCH to select the correct cross-compiled binary.

.gitignore

@@ -51,7 +51,6 @@ QWEN.md
 
 # Local prebuilt binaries
 mpcd-linux*
-Dockerfile.prebuilt
 
 # Dashboard build artifacts
 dashboard/tsconfig.tsbuildinfo

Dockerfile.prebuilt

@@ -0,0 +1,7 @@
+FROM alpine:3.20
+ARG TARGETARCH
+RUN apk add --no-cache ca-certificates
+COPY mpcd-linux-${TARGETARCH} /usr/local/bin/mpcd
+RUN chmod +x /usr/local/bin/mpcd
+EXPOSE 8081 9651 9800
+ENTRYPOINT ["mpcd"]