Bump library/alpine from 3.19 to 3.21

Bumps library/alpine from 3.19 to 3.21.

---
updated-dependencies:
- dependency-name: library/alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Dockerfile

@@ -6,7 +6,7 @@
 # in the Python Docker image we use for the build-stage. The tag of the Python
 # Docker image matches the version of the python3 package available on Alpine
 # for consistency.
-FROM docker.io/library/alpine:3.19 AS compile-stage
+FROM docker.io/library/alpine:3.21 AS compile-stage
 
 ###
 # Unprivileged user variables