Rocker buildpack does not support start script

[craig-willis]

Problem:

I was trying to add an env var via the "start" file (https://repo2docker.readthedocs.io/en/latest/config_files.html#start). I added "start" to the workspace (/WholeTale/workspace/start, added an env var (export PRIVATE_KEY="abcde"), and started an Rstudio capsule. It does not seem to be parsed - R doesn't pick up anything and it's not a variable in the environment in the Terminal.

When we override render() in the Rocker buildpack, we've replaced get_start_script() with start.sh to configure rstudio-server (repo2docker_wholetale/rocker.py#L282, repo2docker_wholetale/r/start.sh). This means that it isn't possible to set env vars via the r2d config as documented.

Test

• Create file start with export PRIVATE_KEY=abcde
• Build/start Rocker RStudio tale
• Open terminal and see if it's set