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

README.md

@@ -88,7 +88,7 @@ or with 1 when images were still being processed after those 5 seconds or
 another part of pixa failed to stop. A request not finished by then is cut off.
 `docker stop` waits 10 seconds before it kills the container.
 
-Outside Docker, pixa needs libvips (the image has 8.15) and libheif to run, as
+Outside Docker, pixa needs libvips (the image has 8.16) and libheif to run, as
 it uses libvips through CGO. pixad does not start unless libvips has its JPEG XL
 support, which on Alpine is the `vips-jxl` package and which the nix and brew
 packages of libvips include, as do the apt ones from Debian 12 and Ubuntu 24.04