Coerce all manager caches to /tmp/containerbase

[rarkins]

We need to support two different read-only FS scenarios:

Full install at build time, no dynamic installs: in this case /opt/containerbase will be fully prepopulated, so cannot be mapped to a Volume, so needs to be read-only after build.

No/minimal install at build time, all dynamic installs: in this case /opt/containerbase should be mapped to a Volume and assumed to be empty at the beginning.

To support the first one in particular, we should coerce all package manager caches to a location in /tmp, proposed: /tmp/containerbase/cache. We should also assume in both scenarios above that either /tmp or /tmp/containerbase is mapped to a Volume and empty to begin with.

Ideally we can coerce all package managers to use our custom cache location through environment variables, and then Renovate does not need to set/override these when calling package managers through child processes.

[rarkins]

Past issue: #2713