dgoss: add support for podman as the container backend

[thrix]

Checklist

• [x] make test-all (UNIX) passes. CI will also test this
• [x] unit and/or integration tests are included (if applicable)
• [x] documentation is changed or added (if applicable)

Description of change

If podman is available on the host, let's use it instead of docker. Podman does not need a daemon to run and is common in RHEL-like OSes.

To be able to use podman as the container backend podman run needs to run a command which does not finish. Commonly sleep infinity is used for that.

Provide enough documentation to understand the feature for the users.

I believe keeping just one command dgoss is a better approach, let's not complicate things for users.

Partially resolves #507

[ekelali]

I would suggest the default continues to be docker. Those that want to use podman can set CONTAINER_RUNTIME to override.

Changing the default would be a breaking change for dgoss consumers that happen to have podman installed.

[thrix]

@ekelali ack, let's mitigate breaking changes

[thrix]

@ekelali it should be resolved

[thrix]

@aelsabbahy hi, do you think we could get this in, or should I do some more changes?

[aelsabbahy]

Sorry, the travis-ci OSS credits ran out again, I had to open up a support ticket. To get the job to trigger, I had to close/reopen the issue.