feat(Dockerfile): add healthcheck

Add a healthcheck command which enables docker compose configurations
using the gateway image to wait for the service to be healthy. This is
done mirroring the existing `wget` command used in our boilerplate.

Dockerfile

@@ -24,4 +24,5 @@ COPY --from=frontend-builder /usr/src/app/build/ ./frontend/build/
 
 ENV GATEWAY_CONSOLE_UI_DIR=/usr/src/app/frontend/build
 
+HEALTHCHECK --interval=10s CMD wget --spider http://localhost:9001/proxy/status
 ENTRYPOINT ["api-gateway-emulator"]