fix(docker): add Go build cache mount and remove unnecessary -a flag

Add --mount=type=cache for Go build cache to speed up repeated builds.
Remove the -a flag which forces full rebuilding of all packages and is
unnecessary when using cache mounts.

Dockerfile

@@ -36,7 +36,8 @@ RUN npm run build:ts
 
 # Build Go binary (cached unless Go files or generated code changes)
 RUN --mount=type=cache,target=/root/go/pkg/mod \
-  CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main ./cmd/server
+  --mount=type=cache,target=/root/.cache/go-build \
+  CGO_ENABLED=0 GOOS=linux go build -installsuffix cgo -o main ./cmd/server
 
 # Test runner stage - includes Go runtime and test dependencies
 # This stage is ONLY used for running tests, never deployed to production