Don't copy in the config - define a volume instead

[dantownsend]

At the moment the Dockerfile copies in the config file:

piccolo-admin-docker/Dockerfile

Line 7 in 376f0ac

COPY ./config.yaml /code/config.yaml

But what we want is for the user to define their own config file.

We can define a VOLUME which tells the user where we expect the config file to be:

VOLUME /config

When the container starts, if no config file is found, we just default to showing all tables, or raise an error.