Improve Docker configuration for releases

[bielsnohr]

Although the Dockerfile, Dockerfile-phaethon, can in principle take either my development image or Helen's production one as an argument, there are some implementation difficulties I ran into with the previous release:

• The HOME environment variable isn't defined by Helen's image, so this would need to be added as an argument
• The USER argument would also need to be set (done now)
• The sudo package needs to be installed because Helen's image doesn't have it

I see a few options forward:

1. Make the modifications above
2. Create my own production (i.e. METHOD=opt) Docker image
3. Make my development image a dual opt and devel one