docker context support

The current docker context isn't respected (#583).

You can work around this by setting DOCKER_HOST before running act, with e.g:

export DOCKER_HOST=$(docker context inspect --format '{{.Endpoints.docker.Host}}')

If your other host requires client certificates, the DOCKER_CERT_PATH environment variable is respected.

export DOCKER_CERT_PATH=$(docker context inspect --format '{{.Storage.TLSPath}}')/docker