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.

mariadb/Dockerfile

@@ -2,7 +2,7 @@ FROM mariadb:latest
 
 LABEL maintainer="Mahmoud Zalt <mahmoud@zalt.me>"
 
-ADD my.cnf /etc/mysql/conf.d/my.cnf
+COPY my.cnf /etc/mysql/conf.d/my.cnf
 
 CMD ["mysqld"]