It would be nice to have possibility to set config path with environment variables.

In my case this would be useful for running migrations in docker container. Environment variables are most common and easiest way to configure containers. That's why i would prefer to use ALEMBIC_CONFIG environment variable than alembic -c ${ALEMBIC_CONFIG} which is painful to pass such command.

It's similar like in docker-compose that you can specify compose file with -f option or with COMPOSE_FILE environment variable.

Here's my PR for this #607