Add option to mount a directory inside SDK harness containers

While experimenting with the Python SDK locally, I found it inconvenient that I can't mount a host directory to the Docker containers, i.e. the input must already be in the container and the results of a Write remain inside the container. For local testing, users may want to mount a host directory.

Since BEAM-5288 the `Environment` carries explicit environment information, we could a) add volume args to the `DockerPayload`, or b) provide a general Docker arguments field.

Imported from Jira [BEAM-5440](https://issues.apache.org/jira/browse/BEAM-5440). Original Jira may contain additional context.
Reported by: mxm.