Update README to clarify docker setup

[harrlight00]

I was implementing this using a docker build, and I ran into an issue that took me a while to debug, so wanted to add my knowledge here.

In addition to adding

# Copy the two files in place and fix different path/locations inside the Docker image
COPY root-config /root/
RUN sed 's|/home/runner|/root|g' -i.bak /root/.ssh/config

to the Dockerfile, you should also utilize an ssh mount with whichever command accesses the private repo. For my case, using go, I changed

RUN go mod download

to

RUN --mount=type=ssh \
   go mod download

[harrlight00]

This is in addition to the necessary changes in the github actions file.

The code I needed was linked in the README (https://docs.docker.com/engine/reference/commandline/buildx_build/#ssh), but might be helpful to add a note in the README.

[LastStarDust]

On top of the --mount=type=ssh option I needed these lines in the Dockerfile:

RUN --mount=type=ssh mkdir -p -m 0700 ~/.ssh && \
    rm -rf ~/.ssh/known_hosts && \
    ssh-keyscan github.com >> ~/.ssh/known_hosts

Moreover, if the user running these commands is different from root, the --mount=type=ssh option must be complemented by --mount=type=ssh,uid=X,gid=Y where X is the correct UID and Y the GID.