Ensure that the configuration file is an absolute path in Docker build

[hhromic]

PR #302 broke the Docker build script as previously documented. See below.

The specific problem is in commit 2ddd7c1, where the passed config file (using the -c option) is now mounted inside the container using the --volume src:dest:opt Docker option.

If you pass a relative-path config file (like in the non-working example you gave in the PR description) you get this error after building the Docker image and starting the pi-gen build:

bash ${RPi-Distro/pi-gen/build-docker.sh} -c config
(...)
./build.sh: line 137: source: /config: is a directory

The problem is that Docker requires absolute paths for mounting single files inside the container, otherwise it silently tries to mount a volume name instead as an empty directory. Therefore the Docker build no longer works with the following invocation forms (relative config-paths):

./build-docker.sh -c myconfig
/path/to/build-docker.sh -c myconfig   # also doesn't work

And instead the Docker build must now be invoked using absolute config-paths:

./build-docker.sh -c /path/to/myconfig   # works
./build-docker.sh -c $PWD/myconfig   # alternative
/path/to/build-docker.sh -c /path/to/myconfig   # another alternative

This is not really fixable because is just the way Docker works when using --volume.

Instead, this PR improves the documentation overall and adds how build-docker.sh works currently.

Update: Fixed now using realpath (see commit 4fd20bd) that preserves old behaviour.

Also I'm silencing a new shellcheck warning and slightly fixing a missing && in a Docker pipeline.

[XECDesign]

A few merge conflicts that need to be resolved

[hhromic]

I have a better idea now to solve the issue addressed by this PR: use realpath to resolve the config file if given as relative. This way it will preserve the expected behaviour. I will be updating the PR in coming days after testing. Please keep it open.
Thanks!

[hhromic]

@XECDesign I rebased and changed now the PR to use realpath in the Docker build script. In this way, the old behaviour is preserved therefore no extra documentation is necessary. It just works.
I tested this and is working fine again for all cases including relative config file arguments.
Hope you are fine with it otherwise let me know of any changes desired.

[XECDesign]

Thanks again, Hugo. Much appreciated.