Add info about alpine base image

For todo in #558

README.md

@@ -19,6 +19,22 @@ The containers being proxied must [expose](https://docs.docker.com/reference/run
 
 Provided your DNS is setup to forward foo.bar.com to the a host running nginx-proxy, the request will be routed to a container with the VIRTUAL_HOST env var set.
 
+### Image variants
+
+The nginx-proxy images are available in two flavors.
+
+#### jwilder/nginx-proxy:latest
+
+This image uses the debian:jessie based nginx image.
+
+    $ docker pull jwilder/nginx-proxy:latest
+
+#### jwilder/nginx-proxy:alpine
+
+This image is based on the nginx:alpine image.
+
+    $ docker pull jwilder/nginx-proxy:alpine
+
 ### Docker Compose
 
 ```yaml