Missing documentation for using Docker build action

Hello. 😄

Thanks for providing the Docker actions to create proper build environments!

I've successfully created a new Docker image on Ubuntu 20.04.3 using the action, but what is the intended/proper way to provide the ltfs source code to the image/container?

I've built the docker image by executing this in the Ubuntu2004-Build folder:
$ docker build -t ltfs-ubuntu .

When I try to run or enter the container, I always get the following error:

/entrypoint.sh: 3: ./autogen.sh: not found
/entrypoint.sh: 4: ./configure: not found
make: *** No targets specified and no makefile found.  Stop.
make: *** No rule to make target 'install'.  Stop.

Makes perfect sense, because the source is not inside the image, nor is WORKDIR set.
I'm rather new to Docker, and I'd be very grateful if you could add usage instructions to the README.md.

Thanks in advance!

Beginne mit README.md und dem Ordner Ubuntu2004-Build, überprüfe anschließend den docker build-Befehl und die Fehler aus entrypoint.sh. Dokumentiere, wie der LTFS-Quellcode dem Image oder Container bereitgestellt wird und wie das Arbeitsverzeichnis festgelegt wird, damit die gezeigten Build-Befehle erfolgreich ausgeführt werden können.