Composer version issue

[renakdup]

Hi, I would like to say thank you for your work.
I found a bug while I tried to build a project locally. It has happened because in your dockerfile you use the latest composer version, but composer.lock requires 1.0 composer API version.
You can see the problem on the picture:

The best way to fix this problem would be to recreate the git tree of the master branch, because this problem is solved at the middle of repository, but I think you will not accept this PR from me.
Anyway, I've fixed it in PR as a hotfix at the end of the commit tree (only a single commit), and I would be happy if you accept it.