Update docker.io/nginxinc/nginx-unprivileged Docker tag to v1.28 (#474)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

Dockerfile

@@ -22,7 +22,7 @@ RUN wkhtmltopdf --outline-depth 4 \
     --enable-internal-links --enable-local-file-access \
     /pdf/index.html /pdf.pdf
 
-FROM docker.io/nginxinc/nginx-unprivileged:1.27-alpine
+FROM docker.io/nginxinc/nginx-unprivileged:1.28-alpine
 
 LABEL org.opencontainers.image.title="puzzle.ch's Application Migration and Modernization Techlab" \
       org.opencontainers.image.description="Container with puzzle.ch's Application Migration and Modernization Techlab content for https://amm-techlab.puzzle.ch/" \