ansible/ansible

Fixes #23095

Fixed issue preventing a caller from setting a docker
configuration file (e.g. docker.yml) by specifing the
config file path in the `DOCKER_CONFIG_FILE` environment
variable. Previously, the cli argument parser set a default value for
the `--config-file` argument which would prevent ever checking the
environment variable, regardless of whether or not the `--config-file`
argument even specified a valid file.

This commit adds a global `DEFAULT_DOCKER_CONFIG_FILE` variable, which
points to the current default `docker.yml` config in the
contrib/inventory directory. Now, when this script is called from the
command line, the config file passed with the cli `--config-file` arg
will be given the highest precedence; if it is absent, this script will
then check if the `DOCKER_CONFIG_FILE` env var is set, and load the
config file specified if possible. If neither the environment variable
or cli argument are specified, then the script will attempt to parse the
config file `docker.yml` in this script's directory (if present).

If either the `DOCKER_CONFIG_FILE` environment variable or the
`--config-file` argument are given but point to a nonexistant file, then
the script will print an error message and exit with an error code.

It is *not* an error condition if the fallback `docker.yml` does not
exist.