Add documentation on enabling login capability about docker-pi-hole

[rosshettel]

It would be great if this could be included as a environment variable so the password would persist on container restarts.

[diginc]

@rosshettel I've debated having a default random password like the new pi-hole installer does. I ended up leaning towards these implementation details:

• Default: random password prints during the 1st boot of container
  • Users check their docker log for the line that says 'password set to' to find new random password.
  • Users could just override with a docker exec pihole pihole -a -p <foobar>
• env var web_password: when null / empty nullifies the need for a password
  • not null value sets the password to that value on first boot.

[middleagedman]

You know.. strange thing is I think it is stored somewhere. I have my /etc/pihole directory mounted from my container, and I've destroyed and rerun the image many times, and somehow that password still sticks.

[AzureMarker]

It's stored in setupVars.conf as a hash.

[middleagedman]

That's a great idea.

[diginc]

Merged in #106