Expanded docker.md documentation

[CampaniaGuy]

Hi, after having initial issues with umap.conf not being found while using docker container i think it could be helpful to add a few more lines in the docker.md for a better start off.

• Added a comment below the umap.conf path definition asserting it is an optional setting
• Specified how to create an admin user and how to access it, otherwise you will experience problems in saving maps or adding tile-layers

[almet]

Thanks for the proposed changes. I've added a few comments, hope they make sense.