This feature allows the filtering of containers with an specific label, those containers that have the filtering label defined would be the ones that would become available for processing.

The feature is fully backwards compatible, if no filtering label is provided via CONTAINER_FILTER_LABEL environment variable all containers in the docker host will be considered for processing.

4 test scenarios has been included, 2 to validate new behavior in both single o multiple instance setups, and 2 to validate the backwards compatible scenarios with the feature not enabled also in both single o multiple instance setups.

The scenarios identified where using this filtering is helpful are:

• Docker engine running huge number of containers not required to be proxied. Shortening the unfiltered container list from the very beginning helps to reduce processing time in further container list iterations.
• Multiple nginx-proxy instances. Assigning different filtering labels to different instances of nginx-proxy allows them to proxy independent services within the same docker engine. This also has the benefit to split responsibilities for scenarios requiring split external and internal facing proxies. In cases where a service is required to be proxied in multiple nginx-proxy instances it suffices only to add all the respective filtering labels to the service's container definition.