Specify fcrepo version in docker-compose.yml

[joecorall]

Without this environment variable set, the milliner container runs logic used against fcrepo v5, but isle-site-template ships with fcrepo 6. Without this environment variable passed to the milliner container, some crayfish derivative services fail in unexpected ways.

Relevant code that sets/references this env var:

• Default value set here
• Environment value referenced here and here