add "npm install && docker install" to container and symlink to tine20 repo

[pschuele]

jcgruenhage:

Ideally it would just be a matter of cloning the repository and running docker build .
the symlink for example could already be present, pointing to ../tine20
if people don't have those repositories next to each other, this would need to be changed, but having a default there that works for most people makes it easier

[pschuele]

maybe we should rename the repo to docker-dev ... another dockerfile/image should be added with prod/test purpose. this might fetch the repo, run npm/composer and build the js (without webpack-dev-server). or just install / contain a defined tarball.