Determine docker host via env then context (#2444)
* Determine docker host via env then context
Previously always used the system default (eg /var/run/docker.sock) which would fail if the default docker socket was not installed, no longer symlinked to the home directory socket, or pointed to a daemon that wasn't running. This can happen when docker desktop and orbstack are running togehter on the same system.
Now cog will try to find the daemon socket by looking at the following in order
1. the DOCKER_HOST env variable
2. the docker context specified by DOCKER_CONTEXT
3. the current docker context set in the docker config
4. the system default
* remove unused code
