update docker readme

[JannikStreek]

For the compose install guide, add a short explanation how to create the db:

docker exec -it mindwendel_db_1 sh
su -- postgres
psql
postgres=# CREATE USER mindwendel WITH PASSWORD 'mindwendel';
postgres=# CREATE DATABASE mindwendel;
postgres=# GRANT ALL PRIVILEGES ON DATABASE mindwendel TO mindwendel;
\q
exit

Add the new env variables:

• DATABASE_SSL: "false"
• MW_DEFAULT_LOCALE: en