Revert to Jessie + init fixes

[djdefi]

Addresses a few outstanding issues.

• Fixes PHP-APC is not included to docker container #57 - Reverted to a Debian Jessie base, using official php and APCu packages
• Closes Include startup actions like "migrate" #36 - On first run, key is generated, database is initialized and /.cachet-installed touch file is added, so that we don't run the php artisan key:generate and php artisan app:install on the second run. Caveat to this is if the Cachet container is deleted (such as on an upgrade) we lose the touchfile. Per the warning in the Cachet install guide: Never change the APP_KEY after installation on production environment. This will result in all of your encrypted/hashed data being lost. -- May need to clarify what that data is that is lost. Another option (probably the right option) is to query for an appropriate value in the database.
• May also address Error 500 after restart the container #34 - I am able to restart the container consistently.
• Adds a check for DB reachability to prevent race conditions.