Add support for .dockerignore files

[fho]

All baur tasks that we execute are run via Dockerfiles.
This gives the opportunity to limit the files that can be accessed in the docker container to the baur inputs.
Ideas

• generate .dockerignore file based on the the inputs of an app + a given docker context path
• generate the baur inputs from a .dockerignore file and a docker context path

Related #141

[fho]

.dockerignore are only used when the files from the context are transferred on docker build.
When running tasks via docker run, required files are normally mounted in the container, .dockerignore files have no effect.

This is not sufficient.