Router development is not currently possible in govuk-docker

[KludgeKML]

The main branch of govuk-docker is now M1-compatible, and part of that is removing the mongo-2.6 container and pointing router to mongo-3.6. This will work for general use, but the router-3.6 container in govuk-docker is not set up to use a replica set, which docker relies on to detect updates to the data. This will causes failures in the following situations:

• developers actively working on router will not be able to run the test suite
• developers using the stack in govuk-docker who publish new content and expect it to appear in their local dev environment through router (rather than directly accessed) will not see that new content.

This issue is related in part to this one: alphagov/router#210

Developers needing a workaround should try this govuk-docker branch which sets mongo-3.6 to use the replica set:
https://github.com/alphagov/govuk-docker/tree/m1-compatibility-keith

Alternatively, work in this router branch will render this replica-set requirement moot when complete:
https://github.com/alphagov/router/tree/change-stream

(This issue repeated in router since it's not 100% clear where it should be, since the fix is in both places: alphagov/router#267)