Add DB migration to Docker entrypoint.

[kobaj]

This is an extension of this PR: #1

My adjustments

• db/migrations/2018_10_01_000000_create_users_tables.php -> Kept failing for me, not sure why or how. Made it slightly more resilient with some checks to see if the table already exists. If there is a way to use Illuminate instead to do an "IF NOT EXISTS" check, that would be better. I just don't know Illuminate.
• docker/entrypoint.sh -> added some log messages!
• startup.sh -> Removed the extra migrate now that it runs in entrypoint. At least, in theory. I don't really know Docker either and have never run startup.sh

[chipuni]

I definitely approve the guards to make sure that tables exist before writing to them.

I don't care where the migration is done.

Approved.