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

Dockerfile

@@ -77,9 +77,11 @@ RUN version="${VERSION:-$(git describe --tags --always)}"; \
         -o /pixad ./cmd/pixad
 
 # Runtime stage, and the last one: a plain `docker build .` builds this
-# stage and what it depends on, and nothing else.
-# alpine:3.21, 2026-02-25
-FROM alpine:3.21@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
+# stage and what it depends on, and nothing else. It must use the Alpine
+# release the golang image above is based on, so that pixad runs against
+# the libvips and musl it was built with.
+# alpine:3.22, 2026-10-08
+FROM alpine:3.22@sha256:5291449c3df73caf6ed85e649dec1b9e818b39a5d8c871e97afc13e9cd5e8fa8
 
 # Install runtime dependencies only. vips-jxl is libvips' JPEG XL
 # support, without which pixad does not start.