Docker hub automated build depend on another docker hub repo

This can be done with DockerHub:

You need to go to the build settings of Test/depend, and look for the section Repository Links. It says:

Link your Automated Build to another Docker Hub repository, and when that repository is updated, it will automatically trigger a rebuild of this Automated Build.

So you just have to add Test/main int the box an you good to go!

I would like to add important information to the answer above.

Repository links do not work with images from the docker library (all image names without a slash / beginning with _/). Apart from that several issues arised in the past which were not taken seriously by Docker Inc, so far (for some of them this means years).

There are multiple issues on GitHub (docker/hub-feedback#518, docker/hub-feedback#1587, partly docker/hub-feedback#1717, docker/hub-feedback#945, docker/hub-feedback#586). In the meantime they added a small hint (that links do not work for official base images) but it is still possible to enable it when your base image is an official image, without any further warning. The hint can only be seen when hovering over the decent info icon.

Some of the issues referenced also indicate that repository links for non official repositories did not work reliably in the past and Docker Inc wasn't very responsive. I recommend to NOT rely on repository links on Docker Hub.