chore(deps): bump alpine in /build in the docker-images group

Bumps the docker-images group in /build with 1 update: alpine.


Updates `alpine` from 3.17 to 3.21

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: docker-images
...

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

Author: dependabot[bot]

build/Dockerfile

@@ -102,7 +102,7 @@ USER 101
 
 
 ############################################# Base image for Alpine with NGINX Plus ##############################################
-FROM alpine:3.20@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a AS alpine-plus
+FROM alpine:3.21@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 AS alpine-plus
 ARG NGINX_PLUS_VERSION
 ARG PACKAGE_REPO
 
@@ -135,7 +135,7 @@ RUN --mount=type=bind,from=alpine-fips-3.20,target=/tmp/fips/ \
 
 
 ############################################# Base image for Alpine with NGINX Plus, App Protect WAF and FIPS #############################################
-FROM alpine:3.17@sha256:8fc3dacfb6d69da8d44e42390de777e48577085db99aa4e4af35f483eb08b989 AS alpine-plus-nap-fips
+FROM alpine:3.21@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 AS alpine-plus-nap-fips
 ARG NGINX_PLUS_VERSION
 ARG NGINX_AGENT
 ARG NGINX_PLUS_VERSION
@@ -172,7 +172,7 @@ RUN --mount=type=bind,from=alpine-fips-3.17,target=/tmp/fips/ \
 
 
 ############################################# Base image for Alpine with NGINX Plus, App Protect WAFv5 and FIPS #############################################
-FROM alpine:3.17@sha256:8fc3dacfb6d69da8d44e42390de777e48577085db99aa4e4af35f483eb08b989 AS alpine-plus-nap-v5-fips
+FROM alpine:3.21@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 AS alpine-plus-nap-v5-fips
 ARG NGINX_PLUS_VERSION
 ARG NGINX_AGENT
 ARG NGINX_PLUS_VERSION