Allow `deno install` when running as `deno` user

Probably the `DENO_INSTALL_ROOT` will need to be changed to a folder where the non-root user has permission to write. The `deno` binary could be installed to `$DENO_INSTALL_ROOT/bin` instead and this folder added to `PATH`.

It could probably be `/opt/deno`: https://unix.stackexchange.com/a/20668/427940

Start with the repository's Dockerfiles and trace the deno user's environment, including DENO_INSTALL_ROOT and PATH. Verify the proposed image behavior by running deno install as the deno user; done means the binary installs into a writable location and is available through PATH.

Written by the indexing model from the issue text.