misnamed .env var

[prash3r]

docker manual in README.md states to set the env var
export MVENTORY_SECRET=<some random string>
it should state instead:
export MVENTORY_SECRET_KEY=<some random string>

EDIT:
MVENTORY_DB_NAME needs to be set as well for connectiong to a mariadb instance via the mysql engine

[proffalken]

Fixed