Only change log-error setting if /etc/my.cnf is not a mount

If the file is mounted the command to comment out the setting can
fail, causing container failure. Also, we do not wish to override
user customizations.

Author: ltangvald

changelog

@@ -4,7 +4,8 @@ Changes in 1.1.1 (not released)
    but should be done for all client commands, or group replication will fail
  * Add flag for logging to console (#87458)
    Setting -e MYSQL_LOG_CONSOLE=true will make the server log to console, so the
-   server log is available through Docker's log interface.
+   server log is available through Docker's log interface. If /etc/my.cnf is
+   mounted from the host system, this has no effect.
  * Ensure both MYSQL_USER and MYSQL_PASSWORD are set (#86982)
    Before, setting only one would cause the option to be silently ignored.
  * Read password file first and throw error if it is empty (#87025)

template/docker-entrypoint.sh

@@ -47,7 +47,10 @@ if [ "$1" = 'mysqld' ]; then
 	SOCKET="$(_get_config 'socket' "$@")"
 
 	if [ -n "$MYSQL_LOG_CONSOLE" ] || [ -n "%%DEFAULT_LOG%%" ]; then
-		sed -i 's/^log-error=/#&/' /etc/my.cnf
+		# Don't touch bind-mounted config files
+		if ! cat /proc/1/mounts | grep "etc/my.cnf"; then
+			sed -i 's/^log-error=/#&/' /etc/my.cnf
+		fi
 	fi
 
 	if [ ! -d "$DATADIR/mysql" ]; then