Let the daemon receive docker stop's signal itself (closes #33)

entrypoint.sh now switches to the routewatch user with setpriv instead of
runuser. setpriv replaces itself with the daemon, so the daemon gets the stop
signal directly and has its full 60 seconds to shut down; runuser stayed in
between and killed the daemon 2 seconds after passing the signal on. setpriv
keeps the environment, so GOMEMLIMIT, MALLOC_ARENA_MAX and XDG_DATA_HOME still
reach the daemon, and the state directory stays
/var/lib/berlin.sneak.app.routewatch.

Model: opus-5-5

Dockerfile

@@ -79,8 +79,8 @@ RUN chown -R routewatch:routewatch /app
 ENV XDG_DATA_HOME=/var/lib
 
 # Cap the Go heap at 1.5 GiB so the runtime collects harder before the
-# container's memory limit is reached. runuser preserves this the way it does
-# XDG_DATA_HOME above.
+# container's memory limit is reached. setpriv in the entrypoint preserves this
+# the way it does XDG_DATA_HOME above.
 ENV GOMEMLIMIT=1536MiB
 
 # Cap glibc's malloc arenas. The SQLite C library allocates and frees millions