Remove Docker hub dependency in tests

[themkat]

See #244. Docker hub has a pull quota for images, about 100 in 6 hours. This can cause tests to fail. Experienced it locally when I got acquainted with the source code (modifying code, running tests, deleting images, rinse and repeat 😛 ). Unsure if the tests run on the same machines with the same IPs in CI, but it could in theory happen there as well.

Believe this, combined with the PR @TomasTomecek is currently doing, #269, will make contributions more pleasant 🙂 Green pipeline and zero random Docker hub related issues are a good thing after all.

Criteria used to select replacement images (all from Quay.io):

• Not too big in size! Should be quick'ish to download.
• Reputable organization or project creating the images. Unsure if pushing the same tag twice is allowed at Quay, but untrusty organizations could in theory do such a thing and add weird stuff.
• Not too many security warnings on Quay.io
• As little changes to existing tests as possible.

[TomasTomecek]

Wow, I thought it would be way harder.

I wasn't aware of the biocontainers org, briefly checked it and looks okay. Since it's CI, we can easily change it if it's unstable.

Thank you so much!