fix(docker): ensure $HOME is set consistently for all configs.

Node uses a .cache directory in $HOME. The default node user has this
set to /home/node, a randomly invented user has / under Docker and
nothing under Podman.

If we set it manually (to /) we get to choose a fixed location and
ensure it's writable for all users.

Dockerfile

@@ -103,6 +103,9 @@ RUN corepack install && \
 
 USER node
 
+# Picked up for Node's .cache directory.
+ENV HOME=/
+
 EXPOSE 6246
 
 VOLUME [ "/opt/data" ]