Custom character set (and set default to utf8mb4)

[edwardsamuel]

By default, the current docker image supports utf8 charset. Unfortunately, utf8 in MySQL is not real UTF-8. utf8 is only support up to 3-bytes per character. The actual UTF-8 is up to 4-bytes per character (e.g: emoji, astral symbol, wider CJK characters, etc.). To full support UTF-8, we can use utf8mb4 charset.

Using the current images, I can't force the MySQL to use utf8mb4 because there is a config to force using utf8 which is not compatible with utf8mb4 (it made my apps broken ☹). utf8mb4 is backward compatible with utf8.