Incompatibility with newest mysql container

[schlaubinski]

I have been using the "tvelocity/etherpad-lite" container for a while now. But when I tried to set it up on a new mashine, I noticed that the "Quickstart" guide does not work any more.

Doing the following
$ docker network create ep_network
$ docker run -d --network ep_network -e MYSQL_ROOT_PASSWORD=password --name ep_mysql mysql
$ docker run -d --network ep_network -e ETHERPAD_DB_HOST=ep_mysql -e ETHERPAD_DB_PASSWORD=password -p 9001:9001 tvelocity/etherpad-lite
results in a crashing etherpad container.

When you strap the -d, you see the following error message:
ERROR 2059 (HY000): Authentication plugin 'caching_sha2_password' cannot be loaded: /usr/lib/mysql/plugin/caching_sha2_password.so: cannot open shared object file: No such file or directory
Looking this up online reveals that this problem has been introduced with the switch from MySQL 5.7 to 8.0 in April 2018. Since no version was specified in the command given in the Quickstart, "latest" is assumed.

A working quickfix is to specify the compatible MySQL version instead:
$ docker run -d --network ep_network -e MYSQL_ROOT_PASSWORD=password --name ep_mysql mysql:5.7

[schlaubinski]

I also provided a pull request with the described quickfix:
Update README.md