Containers fakeroot

[qrplo]

Proposed changes

Improve documentation for containers in mahti/puhti.

• Brief description of how to build containers in the cluster itself by using fakeroot
• Table of some common base images and whether they can be used at all as a base when building images with fakeroot in the cluster

Preview: https://csc-guide-preview.rahtiapp.fi/origin/containers-fakeroot/computing/containers/creating/

• Add note about some folders being mounted by default

Preview: https://csc-guide-preview.rahtiapp.fi/origin/containers-fakeroot/computing/containers/run-existing/

Briefly describe the changes you've made here. Remember to add a link to the preview page of your branch.

Checklist before requesting a review

• [x] I have followed the instructions in the Contributing and Styleguide documents.
• [x] My pull request passes all tests.
• [x] I have included a link to the appropriate preview page (select your branch from the list).

[Nortamo]

Add explicit command for building and minimal example def file for e.g ubuntu and one package.

[rkronberg]

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