Container makes /var/lib/pixa usable before starting pixad (closes #159)

The entrypoint now creates /var/lib/pixa if it is missing. When the
directory or one of its top-level entries belongs to another user or
group, it gives the whole tree to pixad (uid and gid 65532); it then
sets the directory's mode to 750 and runs the server as pixad as
before. Data left by a run under another uid is taken over this way.
Only the top level is checked, so a normal start does not walk the
cache; the tree is changed deepest first, so an interrupted start is
finished by the next one.

"Running under upaas" in README.md no longer tells the operator to
create or chown the host directory.

Model: opus-5-5

TODO.md

@@ -29,6 +29,13 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-09-29 the container makes `/var/lib/pixa` usable by itself (closes
+  #159): `deploy/docker-entrypoint.sh` creates the directory if it is missing,
+  gives the directory and everything in it to `pixad` when the directory or one
+  of its top-level entries belongs to another user or group, sets its mode to
+  `750`, then runs the server as `pixad`; data left by an earlier run under
+  another uid is taken over this way; "Running under upaas" in `README.md` no
+  longer tells the operator to create or chown the host directory.
 - 2026-09-29 variant content types kept in memory (closes #70):
   `Cache.metaCache` holds the content types of up to 10,000 variants in an LRU
   (`github.com/hashicorp/golang-lru/v2`), filled by `StoreVariant` and by