Run pixad on the Alpine release it is built on (closes #229)

The runtime stage of the Dockerfile moves from alpine:3.21 to
alpine:3.22, pinned by digest. The test phase and the build stage use
golang:1.25.4-alpine, which is Alpine 3.22, so pixad was compiled
against libvips 8.16 and ran against 8.15. All three stages now install
their packages from the same Alpine release, where the runtime packages
keep their names. README.md now says the image has libvips 8.16.

Model: opus-5-5

TODO.md

@@ -30,6 +30,10 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 pixad runs on the Alpine release it is built on (closes #229): the
+  runtime stage of the `Dockerfile` uses `alpine:3.22`, the release of the
+  `golang:1.25.4-alpine` image that the test phase and the build stage use, so
+  all three have libvips 8.16 instead of 8.15 in the runtime image.
 - 2026-10-08 requests no longer wait behind eviction queries that read a whole
   table (closes #227): the new `cache_usage` table holds the total cache usage,
   kept up to date by triggers on `source_content` and `variant_content` in the