Use multi-stage docker build

This PR has two changes:

Set frontend dependencies (and nodemon) to devDependencies. This comes in handy for:
Multi-stage Dockerfiles. The image is built in two separate images: the first builds the image and separates the devDependencies from dependencies. The second is a 'clean' image and copies over only the production dependencies and build artifacts (from dist).

This combined reduces the size of the docker image by quite a lot:

REPOSITORY                                   SIZE
filepizza-current                            211MB
filepizza-multi-stage                        151MB
filepizza-multi-stage-and-no-devdependencies 145MB

kern / filepizza

Use multi-stage docker build #127