Add support for Docker Context

[gesellix]

See https://docs.docker.com/engine/context/working-with-contexts/

We might need to use contexts, because Docker Desktop for MacOS will probably change the default socket location from the well-known /var/run/docker.sock to a location in userspace. See docker/for-mac#6529 for details.
The release notes for Docker Desktop 4.13.0 already mentioned this, but the change was rolled back with Docker Desktop 4.13.1:

By default Docker will not create the /var/run/docker.sock symlink on the host and use the docker-desktop CLI context instead.

[gesellix]

See #385: a simple read-only support will be included with the next release