Fix Docker build: statically link CGO binary for distroless

The CGO-enabled binary was dynamically linked against musl libc from
Alpine, but the distroless runtime image uses glibc. This caused
'no such file or directory' at runtime because the dynamic linker
couldn't be found. Fix by passing -linkmode external -extldflags
'-static' to produce a fully static binary.

Author: SamMorrowDrums

Dockerfile

@@ -13,7 +13,7 @@ RUN --mount=type=cache,target=/var/cache/apk \
 RUN --mount=type=cache,target=/go/pkg/mod \
     --mount=type=cache,target=/root/.cache/go-build \
     --mount=type=bind,target=. \
-    CGO_ENABLED=1 go build -ldflags="-s -w -X main.version=${VERSION} -X main.commit=$(git rev-parse HEAD) -X main.date=$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+    CGO_ENABLED=1 go build -ldflags="-s -w -linkmode external -extldflags '-static' -X main.version=${VERSION} -X main.commit=$(git rev-parse HEAD) -X main.date=$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
     -o /bin/github-mcp-server ./cmd/github-mcp-server
 
 # Make a stage to run the app