Allow to configure Rocker base image and RStudio download url. Fixes #12

[Xarthisius]

How to test

1. Checkout this branch and deploy locally (pip install -e .)
2. mkdir -p /tmp/blah && cd /tmp/blah
3. Run:

   cat <<- EOF > /tmp/blah/environment.json
   {
       "config": {
           "buildpack": "RockerBuildPack",
           "command": "/start.sh",
           "environment": [
               "CSP_HOSTS=https://dashboard.wholetale.org",
               "PASSWORD=password"
           ],
           "memLimit": "2048m",
           "port": 8787,
           "targetMount": "/WholeTale/",
           "urlPath": "",
           "user": "rstudio"
       },
       "icon": "https://www.rstudio.com/wp-content/uploads/2014/06/RStudio-Ball.png",
       "iframe": true,
       "name": "RStudio",
       "public": true,
       "tags": [
           "latest"
       ]
   }
   EOF

4. repo2docker --config <path_to>/whole-tale/repo2docker_wholetale/repo2docker_config.py --user-name rstudio --user-id 1000 --no-run . which will output image name e.g. Successfully tagged r2d-2e1600880746:latest
5. Using the image from previous step run docker run -p 8787:8787 -ti -e PASSWORD=password r2d-2e1600880746:latest /start.sh
6. Go to http://localhost:8787/ and make sure that:
   • it runs R 3.5.1 (blurb in the left panel)
   • it runs RStudio 1.2.679 (Help > About RStudio)
   • File pane has WT icon in breadcrumbs
7. Stop your container
8. Modify environment.json by adding:

         "WT_ROCKER_VER=4.0.2",
         "WT_RSTUDIO_URL=http://use.yt/upload/dbda6b23",
         "WT_RSTUDIO_MD5=805e9a4657b294300f8d427e144c3c6d",

   to the config.environment section (be mindful that the last entry in JSON array cannot end with ,)

9. Repeat 4. and 5. (use new image in 5.)
10. Go to http://localhost:8787/ and make sure that:
    • it runs R 4.0.2 (blurb in the left panel)
    • it runs RStudio 1.3.1093 (Help > About RStudio)
    • File pane has WT icon in breadcrumbs

[Xarthisius]

This looks good. How would you feel about finding another place to host the rstudio images than use.yt?

I'd feel great. Anything in particular?