Fix typo on script

There's a tiny typo on [docker-coala.sh line 26](https://github.com/coala/docker-coala-base/blob/master/docker-coala.sh#L26). There should be `=` (or ` ` but since the whole text uses `=` the first preferred for consistency) in between the flag and the parameter.

Open docker-coala.sh and inspect line 26 alongside the surrounding flag assignments. Confirm the separator between the flag and parameter, then verify that the corrected line matches the file's existing syntax and consistency; done means the typo is fixed without changing other script behavior.

Written by the indexing model from the issue text.