fixed outdated Dockerfile

fixes #531

I tried the initial 'official' nodejs image but ran into problems. I then used jessie because wheezy didn't have npm or something. I also used `nodejs-legacy` rather than `nodejs` because it provides a `node` binary rather than `nodejs` which lead to problems during `npm install`. Bower also needed `--allow-root`, but that's not a problem for a container, is it.