Make waiting for mysql to be up more reliable

The current Dockerfile fails for me because it doesn't wait until mysql is up, so I changed it to actually wait until it's responding, rather than a random amount of time. The `mysqladmin ping` command is specifically intended to do this, and contains a built-in retry mechanism, so no messy loops, variables or conditions are needed. If that fails, it issues an `exit 1` in order to fail immediately, rather than waiting for subsequent commands (possibly added by others in future forks) to fail as a consequence.