Allow to build all formats when building docs from a PR

[njsmith]

We recently asked to have pull-request builds enabled for the trio project. This is awesome!

However, we did run into an issue: the PR build passed, but then after we merged, the actual build failed. (And due to unlucky timing, the broken build got tagged and released, so we had to do a .1 release to fix the docs....)

The issue seems to be that when building the docs "for real", RTD builds multiple formats: html, singlehtmllocalmedia, and epub. But when building a PR, it only builds html. And unfortunately, it was the epub build that broke, so we didn't notice until after merging.

Example passing PR build: https://readthedocs.org/projects/trio/builds/11079471/
Example failing build on master: https://readthedocs.org/projects/trio/builds/11086505/

I would expect that the PR build should exactly match the "real" build, because otherwise the green checkmark on the PR build doesn't actually tell you whether the real build will succeed.

[stsewd]

So, we are building html only for now. This is to save some resources.

[graingert]

@stsewd could you provide a GitHub action or docker image so we are able to replicate the configuration on rtd with our own CI infrastructure?

[humitos]

@graingert Unfortunately, we do not have an easy way to replicate our building environment. We do have a Docker image (https://github.com/readthedocs/readthedocs-docker-images) that's the one used for the builders but it's not in a "ready-to-use" state by other people.

Usually, for debugging something specific in real environment, we suggest to use our development environment (https://docs.readthedocs.io/en/stable/development/standards.html). However, it's not a good fit for a CI infrastructure.

[graingert]

@humitos ah I just had a look and I need to configure readthedocs.doc_builder.environments at the same time. I don't think I can mount my repo and then run "build"

[graingert]

@humitos I'm looking for something I can do docker run readthedocs/ci -v "$(pwd)":/rtd with

or even something like curl -s http://readthedocs.org/builder.sh | sh that will work on GitHub actions