Evaluate how to improve Docker build times in devc when Liberty is in build context

[ericglau]

If you run devc while target/liberty already has a Liberty installation, the entire folder is included in the Docker build context which slows down the docker build a bit.

Currently if the docker build takes a long time, we give a message that suggests using a .dockerignore file.

This issue is to evaluate how the above can be improved. Note that we should not just give a general recommendation to put target/liberty in the Dockerfile, since the user may actually want to includes files from that directory in their Docker image, as per our example here.