Run all make commands in container.

[rfranzke]

From https://github.com/gardener/gardener-extensions/issues/122

All make commands require various dev resources to be available:

• golang
• helm
• various gnu utils such as tar, gzip, tr, base64, cut, find, grep, xargs.
• gingo
• linters.

I think that the most reasonable way to get reproducible builds and things such as make generate is to run those commands in a pre-build container image.