Missing documentation for using Docker build action

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Hello. :smile:

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!

Start with README.md and the Ubuntu2004-Build folder, then review the docker build command and the errors from entrypoint.sh. Document how to provide the LTFS source to the image or container and how to set the working directory so the shown build commands can run successfully.

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