fix(test-684): pin base container version

The dockerfile for the regression test connected to issue 684 used a
rolling tag as base image, making it flaky and fail since it was
introduced.

This commit pins the base image to the digest of bionic-20200219, which,
based on the date of the commit that introduced to the dockerfile would
be the most newest ubuntu build and likely what the "rolling" tag
resolved to back then. Since this also an image from the pre-oci days of
ubuntu, this circumvents a bug in container-diff as well
(GoogleContainerTools/container-diff#389)

integration/dockerfiles/Dockerfile_test_issue_684

@@ -1,3 +1,4 @@
-FROM ubuntu:rolling as builder
+# ubuntu:bionic-20200219
+FROM ubuntu@sha256:04d48df82c938587820d7b6006f5071dbbffceb7ca01d2814f81857c631d44df as builder
 
 RUN apt-get update && apt-get -y upgrade && apt-get -y install lib32stdc++6 wget