Bump library/nginx from 1.21.5-alpine to 1.21.6-alpine in /build (#2387)

Bumps library/nginx from 1.21.5-alpine to 1.21.6-alpine.

---
updated-dependencies:
- dependency-name: library/nginx
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

build/Dockerfile

@@ -18,7 +18,7 @@ RUN apt-get update \
 
 ############################################# Base image for Alpine #############################################
 # docker.io/library/nginx is a temporary workaround for Dependabot to see this as different from the one used in Debian
-FROM docker.io/library/nginx:1.21.5-alpine AS alpine
+FROM docker.io/library/nginx:1.21.6-alpine AS alpine
 
 RUN apk add --no-cache libcap \
 	# temporary fix for CVE-2021-42374