Bump alpine from 3.15.0 to 3.15.3 in /build

Bumps alpine from 3.15.0 to 3.15.3.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

build/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.15.0
+FROM alpine:3.15.3
 
 RUN apk --no-cache add \
     ca-certificates