Allow tox testing inside Docker

[ConorMacBride]

The OpenAstronomy Azure Pipelines tox template supports running tox inside docker containers. "This was originally added to support testing against 32bit linux builds using the manylinux official docker images, so there is specific support for these images."

There does not seem to be any public repos on GitHub using the manylinux option or any of the docker_ options. However, only astrofrog/fast-histogram and astropy/astropy-healpix are using the linux32 option.

[astrofrog]

I wonder if it's sufficient to use the publish workflow for this since this does give us 32-bit builds/tests with minimal hassle. We could always document that as a way to do 32-bit builds.

[ConorMacBride]

For most use cases I think that would be sufficient. There won't be any coverage testing and the tox.ini and pytest configuration in setup.cfg won't be used, but unless a package is doing something special for 32 bit, a simple call to pytest should be fine.