Move nginx installation into another build step

Avoids polluting the main image with nginx's build dependencies

image/base/Dockerfile

@@ -17,6 +17,13 @@ FROM builder AS imagemagick_builder
 ADD install-imagemagick /tmp/install-imagemagick
 RUN /tmp/install-imagemagick
 
+FROM builder AS nginx_builder
+ADD nginx_public_keys.key /tmp/nginx_public_keys.key
+ADD install-nginx /tmp/install-nginx
+RUN gpg --import /tmp/nginx_public_keys.key &&\
+    rm /tmp/nginx_public_keys.key &&\
+    /tmp/install-nginx
+
 FROM discourse/ruby:3.3.6-${DEBIAN_RELEASE}-slim AS discourse_dependencies
 
 ARG DEBIAN_RELEASE
@@ -91,6 +98,8 @@ RUN sed -i "s/^# $LANG/$LANG/" /etc/locale.gen; \
 RUN --mount=type=tmpfs,target=/root/.npm \
     npm install -g terser uglify-js pnpm
 
+COPY --from=nginx_builder /usr/share/nginx/sbin/nginx /usr/sbin
+
 # Copy binary and configuration files for magick
 COPY --from=imagemagick_builder /usr/local/bin/magick /usr/local/bin/magick
 COPY --from=imagemagick_builder /usr/local/etc/ImageMagick-7 /usr/local/etc/ImageMagick-7