Container makes its data directory usable itself (closes #42)

The entrypoint now creates the data directory if it is missing and stops
the start when any step fails, so a failed cd can no longer change the
ownership of some other directory. It already took ownership of the
directory and dropped to the routewatch user; that is unchanged. The
README's upaas section now says only which path to mount.

Model: opus-5-5

entrypoint.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+set -euo pipefail
 
 # glibc silently ignores a malformed MALLOC_ARENA_MAX, so refuse it here.
 if [[ -n "${MALLOC_ARENA_MAX:-}" && ! "$MALLOC_ARENA_MAX" =~ ^[1-9][0-9]*$ ]]; then
@@ -6,6 +7,9 @@ if [[ -n "${MALLOC_ARENA_MAX:-}" && ! "$MALLOC_ARENA_MAX" =~ ^[1-9][0-9]*$ ]]; t
     exit 1
 fi
 
+# Give the data directory to the routewatch user before the daemon starts,
+# whether it is missing, an empty root-owned mount, or holds another uid's files.
+mkdir -p /var/lib/berlin.sneak.app.routewatch
 cd /var/lib/berlin.sneak.app.routewatch
 chown -R routewatch:routewatch .
 chmod 700 .