Clarify reliance on home directory

[theoephraim]

Description

Varlock now looks in ~/.varlock for a config file with a few telemetry related settings. In the future there will likely be more settings there. We will also use the folder for a plugin download cache, and likely other cache data as well.

In some situations (particularly in Docker) there may not be a home directory.

We could certainly make it not crash in this situation, but given that we will be using this directory more in the future, we'll need to figure out something else.

One option is to let the user configure the location of the varlock directory, probably via an env var - (not managed within varlock itself). EG VARLOCK_CACHE_DIR=/some/other/path

Another option is just clarifying this in the docs, and suggesting that users override their HOME directory when they are in this situation. For example in a dockerfile adding ENV HOME=/app/.home