Improve Docker run (envsubst, Alpine, documented env file) (#870)

* Add template for secret.json

- Move to faster and lighter Alpine base image

* Update documentation

* Update documentation

* Fix Github token config for secret.json

* Extend env file for Docker runtime configuration

- Update documentation
- Add gh_token for GH personal access token to secret template

* Change http to https in infoSite

* Update .dockerignore

* Update .gitignore

* Update dockerignore

* Remove ENV directive from Dockerfile

- Environment is needed at runtime, not at buildtime

* Docker: contain secret.json in private/

- Incorporates fix from 7c8b0e3d

* Use localhost in example env

* Use baseUrl in GitHub redirect

* Move GH personal token retrieval up

- To remove duplicate Promise.then()

* Typo in shields.example.env

Dockerfile

@@ -1,3 +1,17 @@
-FROM node:6.4.0-onbuild
-ENV INFOSITE http://shields.io
+FROM node:6.9.2-alpine
+
+RUN apk add --update gettext
+
+RUN mkdir -p /usr/src/app
+RUN mkdir /usr/src/app/private
+WORKDIR /usr/src/app
+
+ARG NODE_ENV
+ENV NODE_ENV $NODE_ENV
+COPY package.json /usr/src/app/
+RUN npm install
+COPY . /usr/src/app
+
+CMD envsubst < secret.tpl.json > ./private/secret.json && npm start
+
 EXPOSE 80