Add support for public registries

[displague]

Currently, docker registry username, password, and host values must be provided throughout the Tinkerbell ecosystem.

While authenticated Docker registries are practical for production deployments, in development and home lab environments this additional configuration and requirement creates a burden and restricts use.

By making these registry arguments optional, we enable:

• local registries without authentication (such as the kind-local registry)
• running workflows using upstream images directly

This may require changes to boots, tink-worker, and perhaps others. This issue only reflects the tink-worker changes.

Originally posted by @detiber in #287 (comment)

[displague]

Also see https://github.com/tinkerbell/tink/pull/287/files#r491163737

[gianarb]

This is in some way related to #134 and #90 . Community is asking for both. Give us a secure environment and remote passwords.

Consistency is the only takeaway here

[detiber]

@gianarb I think it might be tangentially related.

Ideally we'd support working with an existing registry host more easily, which is what this would be targeting. Ideally if we do deploy a registry it should be secure, but if a user already has a registry (or wants to use a public registry), then we should allow for it to be used and not necessarily require authentication for pulls.

[jacobweinstock]

FYI, this ticket and others that are similar have been captured in this discussion #586

[mmlb]

#607 touches on this a little bit

[jacobweinstock]

close by #607