rocker.sh: figure out how to export variables to scripts

[eeholmes]

To prevent " becoming %22 in things like "https://${R_VERSION}", I had to do this var_assignment=$(echo "$line" | sed 's/^ENV //g' | sed 's/"//g')

to get rid of " so this R_VERSION="4.4.1" becomes export R_VERSION=4.4.1

this will cause problems when exporting things which have a $ like in this file. https://github.com/rocker-org/rocker-versioned2/blob/master/dockerfiles/cuda_4.4.1.Dockerfile

figure out a more robust solution

[eeholmes]

eval "export $var_assignment" worked