[docs] Document airgap/offline Che deployment process/options

[amisevsk]

Is your task related to a problem? Please describe.

Issues #14733, #14733, eclipse-che/che-plugin-registry#211, and eclipse-che/che-plugin-broker#70 add offline functionality to Che registries and workspaces. This functionality is going to be supported by the operator (#14734) and by chectl (#14699)

We should add a new section to the Che documentation on how to use these options and how to deploy Che in an airgap environment.

[davidfestal]

How does this issue relate to the following: #14088 (comment) ? Maybe it should replace it ?

@l0rd @nickboldt wdyt ?

[nickboldt]

There are two types of doc required here:

a) how do I build an airgap-compliant image?
b) how do I use a previously-built airgap-compliant image?

Part (a) is about which flags to pass to build.sh or docker build to ensure you enable the offline caching mode when building and swap in the correct overrides for docker.io/quay.io with your own registry URL.

I can see this doc being useful for Che 7 extenders wanting to build custom registries to add/remove new plugins/devfiles. Or for CRW 2 airgap customers wishing the same thing - custom registries with more/less content than the OOTB default

Part (b) is... now that I have a build, how do I tell chectl to use MY images instead of the defaults?

This is also for Che 7 extenders but could also be useful for a CRW 2 customer who just wants to deploy the CRW 2 OOTB experience (non-customized registries) inside an airgap environment.

So it's either two documents or one two-part document. Given you might only need the second doc if you're not looking to customize registry content, I might suggest it's better as two docs.

[amisevsk]

Draft document: https://gist.github.com/amisevsk/c98a48bba01f3fa72d7ab7dfea45fc36

There are a few known issues and still some work required in filling in various options (e.g. how to deploy using chectl).

[amisevsk]

PR merged.