Allow running docker container as non-root

Related issue: #1334

This doesn't solve issue, it just allows running without root with almost no workarounds.
I'll probably play with it more later, but now it's needed to make my instance work again.

There's no need to copy frontend if /frontend directory doesn't exist:
We want to copy only if it's mounted outside of container and when it's mounted it exists.
Creating it needs root and is not needed.

Entrypoint directory wasn't readable nor listable by non-root user in some latest containers.