[Elia]

I got 40 tinkering, and i go to containers and it only shows metal box, metal chest, metal golden chest..
lol

The crafting list of items shows everything you're able, or will be able to make, regardless of your skill.

Any container is lockable, I think your skill as a tinker determines either the chance that you'll make it and it's lockable (has a key inside after being made) or it determines how easy it is to lockpick.

Yeah, tranith, and the easiness of being lockpicked doesn't affect the skillgain, so you can train it with minimum Tinkering. Even if you'll be able to lockpick it from the first try (which I doubt), your skill will rise.