Add Apptainer (Singularity) to Gitpod nf-core base image

[MatthiasZepper]

To test and develop the respective features for #2336, I had to install Apptainer in Gitpod several times a day.

Although I am not sure how often other people will need it, I see little harm in adding Apptainer by default to the Gitpod image. The image is already quite bloated with conda, so the additional 80MB-100MB will barely matter?

Unfortunately, I could not test the new image. I tried to build it locally, but the RUN nextflow self-update step seems to get stuck at Downloading dependency org.pf4j:pf4j-update:jar:2.3.0. That one step is running now since 5h and the whole build so far is taking a little over 7h, which I think might be a MacM1 issue?

So whoever will review this PR should please clone this, navigate to the folder cd tools and run

docker build . -f nf_core/gitpod/gitpod.Dockerfile -t testgitpodwithapptainer

to see if the image builds successfully and how big the new image will be.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #2345 (ae53738) into dev (b5eb200) will decrease coverage by 0.39%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #2345      +/-   ##
==========================================
- Coverage   73.08%   72.69%   -0.39%     
==========================================
  Files          78       78              
  Lines        8784     8864      +80     
==========================================
+ Hits         6420     6444      +24     
- Misses       2364     2420      +56     

see 2 files with indirect coverage changes

[MatthiasZepper]

Thx.

PS: I like indentation too, but one of the code formatters that are run on save in my editor (prettier? black?) didn't.