Throw error with explicit latest tag

[faust64]

following-up on #360

latest tag handling.

Fixes #321

[jakubhajek]

I tested that change locally, no issues occurred.

[SantoDE]

@faust64 what do you think of the comment in #321? Do you think that would be sufficient? :)

[faust64]

Sure, one should never use latest tag. At least when you're not pulling from a registry that expunges older images, or some rare exceptions when you need some missing feature that's not in an official release, ... doesn't really apply here.
Then again, good practices are rarely observed.
Another take on it may be to throw an error instead.

[SantoDE]

Like to throw an error if you are trying to run it with latest? I would like that :)

[faust64]

allright, that should do it

[jakubhajek]

It works as expected preventing a user to deploy the latest tag.

[ldez]

LGTM