MYSQL on IPv6 docker network

[moutasem1989]

Creating an IPv6 Docker network, makes MYSQL unreachable to Seafile.
I assume that in bootstrap.py, the User Host should be '%' instead of 'MYSQL_USER_HOST': '%.%.%.%',. Otherwise MYSQL would throw an error „access denied“ when trying over IPv6.

[herver]

Indeed, I've been pulling my hair off for the past couple of hours only to realize this,

I think someone already fixed this exact problem in #288