Fix base image for mysql-8 #3388135

- removed extra layer saved 20MB of apt-cache
- /var/lib/mysql is empty initially

db/mysql-8/Dockerfile

-FROM       mysql:8
+FROM       mysql:8.0-debian
 MAINTAINER drupalci
 
 # Packages.
 RUN apt-get -y update && \
-    apt-get -y install netcat psmisc
-
-RUN apt-get clean && apt-get autoclean && apt-get -y autoremove
+    apt-get -y install netcat psmisc --no-install-recommends && \
+    rm -rf /var/lib/apt/lists/*
 
 COPY ./conf/my.cnf /etc/mysql/conf.d/drupalci.cnf
 
-RUN rm -rf /var/lib/mysql/*
-
 USER root
 
 COPY ./conf/startup.sh /opt/startup.sh