Add snap support for docker interface for app dev builds

[nicktate]

Description

What does this pull request accomplish

Adds app-dev-build plug w/ the docker interface

This will require the user to have docker installed via snap and execute snap connect doctl:app-dev-build docker:docker-daemon to grant permissions

[andrewsomething]

👍 LGTM!

[andrewsomething]

Also wanted to point out that we do have a few places where we do some custom error messages to point snap users in the right direction. E.g.

doctl/commands/registry.go

Lines 425 to 429 in 51c4f6b

	_, isSnap := os.LookupEnv("SNAP")
	if os.IsPermission(err) && isSnap {
	warn("Using the doctl Snap? Grant access to the doctl:dot-docker plug to use this command with: sudo snap connect doctl:dot-docker")
	return err
	}

Might want to consider doing that for app dev builds as well.

[nicktate]

Also wanted to point out that we do have a few places where we do some custom error messages to point snap users in the right direction. E.g.

doctl/commands/registry.go

Lines 425 to 429 in 51c4f6b

	_, isSnap := os.LookupEnv("SNAP")
	if os.IsPermission(err) && isSnap {
	warn("Using the doctl Snap? Grant access to the doctl:dot-docker plug to use this command with: sudo snap connect doctl:dot-docker")
	return err
	}

Might want to consider doing that for app dev builds as well.

💯 @kamaln7 and I were just talking about this as well. I will add it in, thanks for the reference