adds ability to dynamically set config env variable (#2)

Why:
* current setup does not allow for dynamically setting what config to read via Dockerfile directly or via use of env variables in docker-compose file
* given multiple instances/environemts, need to be able to use same Dockerfile to set what config to read dyamically

This change address the need by:
* introducing the use of ARG values in Dockerfile to set value of CONFIG env variable at image build time

Author: msuzoagu

Dockerfile

@@ -10,7 +10,8 @@ ENV PORT=3000
 EXPOSE ${PORT}
 
 #  Which configuration in the config.json file to load
-ENV CONFIG="production"
+ARG CONFIG="production"
+ENV CONFIG=${CONFIG}
 
 # Location of config.json
 ENV CONFIG_DIR=/usr/src/app/config/

README.md

@@ -13,7 +13,7 @@ Flow for automated deployment:
 
 This webhook is intended for use with private Docker Hub repositories and self hosted Docker Swarm instances.
 
-To get started, clone this repository, add an image of it to your Docker Hub account, configure `config.json` and deploy it to your Docker Swarm as a service (see steps below).
+To get started, clone this repository, add an image of it to your Docker Hub account, configure config.json and deploy it to your Docker Swarm as a service (see steps below).
 
 [Read more about this service in this blog post.](https://medium.com/@iaincollins/docker-swarm-automated-deployment-cb477767dfcf)
 
@@ -27,6 +27,14 @@ Supported environment variables:
     USERNAME="docker-hub-username" // A Docker Hub account username
     PASSWORD="docker-hub-password" // A Docker Hub account password
 
+**note:** the value for `CONFIG` environment variable can be passed in at image build time:
+
+```
+  docker build \
+   --build-arg CONFIG=part-of-config-dot-json-file-to-load \
+   -t docker-hub-username/docker-deploy-webhook .
+```
+
 The `config.json` file defines each environment:
 
     {