Options for installing extra packages at runtime

[JoshKarpel]

Per #25 , a very common desire will be adding extra packages to the Python environment. We can let people do this without having to create their own Docker images (as we tell them to do right now) by adding some hooks to our entrypoint script.

This PR will implement these hooks:

• conda_env, a full conda environment that the worker environment will be updated with.
• conda_packages, a plain list of conda packages to install.
• pip_packages, a plain list of pip packages to install.

We may, in the future, look into a hook for system packages, as mentioned in the discussion in #25.

[JoshKarpel]

Ok, well, I tried my first version out and there's a big problem: we're going to get hosed by not running as root.

daskdev/dask depends on continuumio:miniconda3, which installs miniconda as root in /opt/miniconda. The permissions are

drwxr-xr-x 1 root root 4096 Jun 19 21:08 conda

which is great because it means that any user can run code, but means that no one but root can install any packages. HTCondor will never run a container as root, so we're in a bind. You would need to build a special image where the conda installation is world-writable to do more installs at runtime, which is precisely what we don't want users to have to do.

One possibility would be to produce our own versions of popular images, which are just one more layer with all of the permissions set appropriately, but that doesn't sound very sustainable.

Back to the drawing board, for the moment....

[JoshKarpel]

Made some progress, but it involves unsetting conda's default package cache, which means it can't re-use already-installed packages. More things to play with...