HTTP proxy support

[donovanmuller]

HTTP proxy does not seem to be supported at the moment. Getting the following error:

Build container for action use: '/runner/_work/_actions/jzweifel/gatsby-cli-github-action/master/Dockerfile'.
  /usr/local/bin/docker build -t 60e226:dbbcc87c8e7d40b4867a5be98d3e8cfa -f "/runner/_work/_actions/jzweifel/gatsby-cli-github-action/master/Dockerfile" "/runner/_work/_actions/jzweifel/gatsby-cli-github-action/master"
  Sending build context to Docker daemon  76.29kB
  
  Step 1/14 : FROM node:10-slim
...
  Status: Downloaded newer image for node:10-slim
   ---> 50b20520e882
  Step 2/14 : LABEL version="1.0.0"
   ---> Running in 4f440084cb01
  Removing intermediate container 4f440084cb01
   ---> 6c464c5383b9
  Step 3/14 : LABEL "repository"="http://github.com/jzweifel/gatsby-cli-github-action"
   ---> Running in 76e072f4ae5b
  Removing intermediate container 76e072f4ae5b
   ---> 88b308d8b2fd
  Step 4/14 : LABEL "homepage"="http://github.com/jzweifel"
   ---> Running in 2387e1ce1983
  Removing intermediate container 2387e1ce1983
   ---> 3ede55b09d66
  Step 5/14 : LABEL "maintainer"="Jacob Zweifel <jacob@jacobzweifel.com>"
   ---> Running in 63acb5674970
  Removing intermediate container 63acb5674970
   ---> e2e59677f4b1
  Step 6/14 : LABEL "com.github.actions.name"="Github Action for Gatsby CLI"
   ---> Running in 727d05686289
  Removing intermediate container 727d05686289
   ---> b4d956499620
  Step 7/14 : LABEL "com.github.actions.description"="Wraps the Gatsby CLI to enable common Gatsby commands."
   ---> Running in 6e164c110a93
  Removing intermediate container 6e164c110a93
   ---> 86e49e8f0069
  Step 8/14 : LABEL "com.github.actions.icon"="package"
   ---> Running in ed147aa2abb0
  Removing intermediate container ed147aa2abb0
   ---> c6af58d312d2
  Step 9/14 : LABEL "com.github.actions.color"="purple"
   ---> Running in 67fcbb529305
  Removing intermediate container 67fcbb529305
   ---> 576e0d592dba
  Step 10/14 : COPY LICENSE README.md THIRD_PARTY_NOTICE.md /
   ---> cbe240243cf6
  Step 11/14 : RUN yarn global add gatsby-cli
   ---> Running in fa0f40fdec6c
  yarn global v1.22.5
  [1/4] Resolving packages...
  info There appears to be trouble with your network connection. Retrying...
  info There appears to be trouble with your network connection. Retrying...
  info There appears to be trouble with your network connection. Retrying...
  Error: The operation was canceled.

The runner (self-hosted) has a proxy configured:

Runner is running behind proxy server 'http://proxy.xxx.com:3128' for all HTTP requests.
Runner is running behind proxy server 'http://proxy.xxx.com:3128' for all HTTPS requests.

and I also tried adding explicit environment variables:

      - name: Build Gatsby Site
        uses: jzweifel/gatsby-cli-github-action@master
        env:
          https_proxy: http://proxy.xxx.com:3128
        with:
          gatsby-arg: build