Building a new proxy image will include the legacy assets

This is meant to address a potential WTF moment if anyone was to build
thechangelog/proxy image on their host without having the legacy static
assets present. The goal is for the legacy assets to be seamlessly
pulled just-in-time, at the time of building the proxy image, without
any extra hoops or WTF moments.

This approach makes use of Docker multi-stage builds:
https://docs.docker.com/develop/develop-images/multistage-build/

The resulting legacy_assets image is available via
https://hub.docker.com/r/thechangelog/legacy_assets/

[finishes #162167357]

Makefile

@@ -94,6 +94,14 @@ contrib: $(COMPOSE) ## Contribute to changelog.com by running a local copy (c)
 .PHONY: c
 c: contrib
 
+.PHONY: legacy-assets
+legacy-assets: $(DOCKER)
+	@echo "$(YELLOW)This is a secret target that is only meant to be executed if legacy assets are present locally$(NORMAL)" && \
+	echo "$(YELLOW)If this runs with an incorrect $(BOLD)./nginx/www/wp-content$(NORMAL)$(YELLOW), the resulting Docker image will miss relevant files$(NORMAL)" && \
+	read -rp "Are you sure that you want to continue? (y|n) " -n 1 && ([[ $$REPLY =~ ^[Yy]$$ ]] || exit) && \
+	cd nginx && $(DOCKER) build --tag thechangelog/legacy_assets . --file Dockerfile.legacy_assets && \
+	$(DOCKER) push thechangelog/legacy_assets
+
 .PHONY: proxy
 proxy: $(DOCKER) ## Builds & publishes thechangelog/proxy image
 	@cd nginx && export BUILD_VERSION=$$(date -u +'%Y-%m-%d') ; \

nginx/Dockerfile

@@ -1,5 +1,6 @@
+FROM thechangelog/legacy_assets AS legacy_assets
 FROM jwilder/nginx-proxy
 
-COPY ./www/wp-content /var/www/wp-content
 COPY ./vhost.d/default /etc/nginx/vhost.d/default
 COPY ./conf.d/changelog.conf /etc/nginx/conf.d/changelog.conf
+COPY --from=legacy_assets /var/www/wp-content /var/www/wp-content

nginx/Dockerfile.legacy_assets

@@ -0,0 +1,2 @@
+FROM scratch
+COPY ./www/wp-content /var/www/wp-content