run php-fpm under workspace user

[lordofthebrain]

Run php-fpm under the workspace user. This solve all permissions problems. All files like cache files will be created with the same user

I have also add -l flag to useradd. When you use laradock with a domain user you have a large uid and your machine will be frezze by useradd without -l flag. usermod has no -l flag so I use a sed hack in php-fpm Dockerfile to avoid the frezze problem.

See: moby/moby#5419 or
https://forums.docker.com/t/run-adduser-seems-to-hang-with-large-uid/27371

I completed the 3 steps below:

• [] I've read the Contribution Guide.
• [] I've updated the documentation. (refer to this for how to do so).
• [] I enjoyed my time contributing and making developer's life easier :)

[jasperf]

Found this an interesting idea as I thought GD Library perhaps had issues resizing images due to permissions so I tried. Longer story at #1969 . Did not see this helping changing image files to being stored with permissions laradock:laradock though. And error 500 remained. So perhaps this pull request is not working this way.

[lanphan]

already have.
Thanks for your PR.