[Self-Hosted] Preflight check for to check for firewall settings

[lucasvaltl]

Is your feature request related to a problem? Please describe

Some customers have their firewall / proxy setup so that it just swallows invalid connections instead of outright denying it (this can also be called stealth mode, silent block / fail). This shows up as a container image taking forever to download, which is in fact blocked.

Describe the behaviour you'd like

A kots preflight or diagnostic that can tell if gitpod (and specifically the workspace nodes) can get https://eu.gcr.io/v2/ within N seconds would circumvent some of these problems

Describe alternatives you've considered

Additional context

Internal slack thread

[lucasvaltl]

I assume we need to make this check dependent on whether we are in an airgapped environment or not?