Give some love to the Dockerfile

[humitos]

It would be awesome to follow some of the tips and tricks to build good Docker images like:

• use multi stage builds
• remove all residuals after apt
• remove pyenv build environments after compiling
• remove all pyenv dependencies installed via apt
• --squash, although it's in experimental :/
• delete miniconda .sh file after installed
• wget master.zip pyenv instead of clone it
• pip cache (--no-cache-dir should be useful)
• more?

Ref: https://docs.docker.com/v17.09/engine/userguide/eng-image/dockerfile_best-practices/

[humitos]

Just built 18.04 image from #59

readthedocs/build  18.04  27e5be0051e1  2 minutes ago  7.33GB

[bebehei]

--squash won't be a good choice. This docker image keeps adding files layer by layer. It does not change, delete or replace any files.

[bebehei]

use multistage builds: This might shave off some hundred MBs in the image.

I'm not deep into the rtd project internals yet. So how many of those -dev packages are superfluous after the build?

[varac]

Yes please, the insane size of the image holds me back from using it, really (readthedocs/build:7.0 is ~3gb big).