Introduce custom base image

To avoid discussions between the mainline guys and stable guys,
this PR introduces customizable base images via build args.

Closes #1189

Dockerfile

@@ -1,4 +1,5 @@
-FROM nginx:1.14.1
+ARG BASE_IMAGE_TAG=1.14.1
+FROM nginx:$BASE_IMAGE_TAG
 LABEL maintainer="Jason Wilder mail@jasonwilder.com"
 
 # Install wget and install/updates certificates

Dockerfile.alpine

@@ -1,4 +1,5 @@
-FROM nginx:1.14.1-alpine
+ARG BASE_IMAGE_TAG=1.14.1-alpine
+FROM nginx:$BASE_IMAGE_TAG
 LABEL maintainer="Jason Wilder mail@jasonwilder.com"
 
 # Install wget and install/updates certificates

README.md

@@ -36,6 +36,24 @@ This image is based on the nginx:alpine image. Use this image to fully support H
 
     $ docker pull jwilder/nginx-proxy:alpine
 
+### Custom base image
+
+nginx-proxy runs with the latest stable nginx. You can build your own image and choose a different base image tag (e.g `1.15.7` or `1.15.7-alpine`).
+
+```
+# debian
+$ docker build --build-arg BASE_IMAGE_TAG=1.15 -t nginx-proxy-test-1.15 .
+$ docker run --rm nginx-proxy-test-1.15 nginx -v
+nginx version: nginx/1.15.7
+
+# alpine
+$ docker build --build-arg BASE_IMAGE_TAG=1.15-alpine -t nginx-proxy-test-1.15 -f Dockerfile.alpine .
+$ docker run --rm nginx-proxy-test-1.15 nginx -v
+nginx version: nginx/1.15.7
+```
+
+For docker-compose see [docs.docker.com/compose/compose-file](https://docs.docker.com/compose/compose-file/#args).
+
 ### Docker Compose
 
 ```yaml