This PR contains a few fixes to the docker client and surrounding code. While testing #2417 I realized tox was blocking the COG_DOCKER_SDK_CLIENT variable in some integration tests, but it was correct on others. I sorted that out so the entire suite is ran against both clients and fixed up some issues that came up.

The actual changes are pretty small, but the diff is huge because I added pexpect to the test group for a new integration test.

Here's what's fixed:

• add a timeout to the predictor health check loop so a hanging model won't block startup
• set the buildkit frontend syntax to the latest dockerfile version
• bind exposed ports to ipv6 in addition to ipv4
• only attach stdin to the container if stdin has data to read (like a file or pipe)
• fix relative path to project root for docker context when running in a sub directory
• fix for additional contexts being available but missing a name for the docker frontend
• smarter handling of interactive/tty handling for stdin (and integration tests!)