fix(#798): Honor DOCKER_HOST environment variable for Docker API client

[james-hu]

Overview

TaskCat fails when running in Kubernetes with contained runtime or in GitLab pipeline due to Docker daemon connection errors.

Testing/Steps taken to ensure quality

See issue #798. As long as the UNIX socket is not available, the error happens.

Solution

Allow users to specify a custom Docker host via the DOCKER_HOST environment variable, which is passed to the Docker APIClient constructor.

Changes

• Docker APIClient now uses DOCKER_HOST if set, otherwise defaults to unix://var/run/docker.sock

Testing Instructions

Test in an environment that the Docker daemon's UNIX socket is not accessible by the client, for example in a GitLab pipleine with docker:dind service.

[james-hu]

I am not familiar with this... what should I do to make "Static Analysis" green?