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
the golang image built on 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. The golang pins now name that release as
golang:1.25.4-alpine3.22, with the same digest, and say that the runtime
stage uses it too. 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-alpine3.22` image that the test phase and the build stage use,
+  so all three have libvips 8.16, where the runtime image had 8.15.
 - 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