Apply COPY vs ADD best practices

Should only use `ADD` for URLs (and even then, `curl` or `wget` are preferred, for numerous reasons, even within this repo) and when extracting archive contents as part of the `ADD` operation; otherwise `COPY` is clearer and (slightly) more efficient.

mysql/Dockerfile

@@ -13,7 +13,7 @@ RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
 
 RUN chown -R mysql:root /var/lib/mysql/
 
-ADD my.cnf /etc/mysql/conf.d/my.cnf
+COPY my.cnf /etc/mysql/conf.d/my.cnf
 
 CMD ["mysqld"]