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.

percona/Dockerfile

@@ -4,7 +4,7 @@ LABEL maintainer="DTUNES <diegotdai@gmai.com>"
 
 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"]