update saf docker install readme instructions

[em-c-rod]

Problem: The previous instructions for installing SAF via docker did not specify an entrypoint, so if someone ran the command, it ran, showed the SAF help results, then stopped and removed the container.

Fix: Adding specification about the entrypoint so people can run the command and stay in the docker container to run more SAF CLI commands.

The windows command does not appear to work in powershell from minimal testing. This needs clarification.

Signed-off-by: Emily Rodriguez [email protected]

[camdenmoors]

What's the advantage of dropping into a shell here? It shows the help because that's just what's shown when we run saf, should we clarify the readme with docker run --rm -it -v$(pwd):/share mitre/saf <topic>/docker run --rm -it -v$(pwd):/share mitre/saf <arguments>?