README.md: added explanation on copying files from Docker image

When building Maneage inside a Docker container, in the end the users want
to extract the final outputs from the container into their host operating
system to inspect more comfortably. So with this commit, a short
examplanation has been added on how to do this.

We also noticed that it is much better if the 'Dockerfile' is stored and
run in an empty directory, otherwise, it will start parsing the full
directory and its subdirectories as the docker image's environment.

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