Update Default credentials in DOCKER.md

[tmartincpp]

Hello,

I hope I don't waste your time this time :)

Description:

I followed the Docker guide available at https://github.com/sourcentis/deming/blob/main/DOCKER.md. However, the default credentials does not seems to work.

Instead I was able to login with the login details from https://github.com/sourcentis/deming/blob/main/INSTALL.md.

Solution proposed:

Modify DOCKER.md to link the Default Credentials section to INSTALL.MD.