Pin fake-gcs to 1.18

services/filestore/test/acceptance/deps/Dockerfile.fake-gcs

@@ -1,4 +1,4 @@
-FROM fsouza/fake-gcs-server
+FROM fsouza/fake-gcs-server:1.18.4
 RUN apk add --update --no-cache curl
 COPY healthcheck.sh /healthcheck.sh
 HEALTHCHECK --interval=1s --timeout=1s --retries=30 CMD /healthcheck.sh http://localhost:9090