ci: install older mysql-server packages

The newer MySQL 8.0 packages on Ubuntu 20.04 contains
some kind of bug that causing issues in our CI.

This pins the MySQL server to an older Ubuntu package
8.0.19-0ubuntu5 that does not have this issue.

We need to report this to Ubuntu packaging.

images/Dockerfile.ci

@@ -37,8 +37,19 @@ RUN apt-get update -y && apt-get install -qy \
         libsnappy-dev \
         libprotobuf-dev \
 # For redis
-        redis-server \
-        && rm -rf /var/lib/apt/lists/*
+        redis-server
+
+# TODO(tobias-urdin): Downgrade MySQL to a version that works. Report this
+# bug to Ubuntu packaging.
+ENV MYSQL_VERSION=8.0.19-0ubuntu5
+RUN apt-get install -qy --allow-downgrades mysql-client=$MYSQL_VERSION \
+        mysql-client-8.0=$MYSQL_VERSION \
+        mysql-server=$MYSQL_VERSION \
+        mysql-server-8.0=$MYSQL_VERSION \
+        mysql-client-core-8.0=$MYSQL_VERSION \
+        mysql-server-core-8.0=$MYSQL_VERSION
+
+RUN rm -rf /var/lib/apt/lists/*
 
 #NOTE(sileht): really no utf-8 in 2017 !?
 ENV LANG en_US.UTF-8