DEV: release Makefile target and documentation

Makefile

@@ -12,3 +12,8 @@ push: build
 .PHONY: build
 build:
 	docker build --pull -t ${IMAGE}:${TAG} .
+
+.PHONY: release
+release:
+	docker tag ${IMAGE}:${TAG} ${IMAGE}:latest
+	docker push ${IMAGE}:latest

README.md

@@ -54,3 +54,17 @@ docker run discourse/auth-proxy
 ```
 
 Running will display configuration instructions
+
+You may build an image using
+
+```
+make build
+```
+
+After building, you may publish it as a new release -- i.e. `latest` tag -- using
+
+```
+make release TAG=20220406.135620
+```
+
+(Replace with the actual built tag.)