Client basic auth support

[ag-TJNII]

This PR adds client auth support via simple HTTP basic auth. The flow here is:

• The admin creates a htpasswd file using normal tools, and volume mounts that into the container
• The CLIENT_AUTH_USER_FILE ENV var is set to the htpasswd file path in the container
• The presence of the CLIENT_AUTH_USER_FILE variable enables the client_auth.conf file in the nginx config, which configures basic auth via ngx_http_auth_basic_module.

To pull from the registry clients must now auth with docker login using credentials from the htpasswd file. Unauthenticated clients will get a error.

When CLIENT_AUTH_USER_FILE is unset (the default) application behavior is unchanged.

This feature is useful in environments where having an open proxy that can pull from ECR is undesirable for security reasons.