Warn if NOT --pull-policy=always in container

[natalieparellano]

Fixes buildpacks/pack#2222

[jjbustamante]

This one it is ok for me, my only doubt was importing the whole kaniko dependency when the code we are using to determine if we are running in a container environment is based on checking the existence of files.

I know this could be weird, but what happen if we just copy their logic?, I do not think many new container environment appears everyday.

[natalieparellano]

my only doubt was importing the whole kaniko dependency

Fair, but we're already having to keep our deps in sync with kaniko because of our dependency on the lifecycle (see buildpacks/pack#1574). As we're only pulling in a relatively small package, it doesn't bother me too much (and I'd like to avoid having to maintain more things). But, happy to reevaluate if there are strong feelings about it.