Prior to this commit, if a DOCKER_HOST environment variable was present
when attempting to communicate with a Docker daemon, it was assumed
that the value of that variable was an address that could be used to
create an HTTP connection to a remote daemon. In some cases, the value
of the variable is the path to a local socket file, which would cause
the HTTP connection to fail.

This commit adds additional validation of the value of the DOCKER_HOST
environment variable to determine whether it is a remote address or
a local socket file and create the appropriate connection type.

Fixes gh-21173