Modifications to install cmdstan that can be accessed by normal user

By default `cmdstanr::install_github` installs cmdstan in
`file.path(Sys.getenv("HOME"),".cmdstan")`. Since the RUN cmd is run as
root cmdstan is installed in `/root/.cmdstan` which is inaccessible to
any other normal user. Since I start the container as a normal user
cmdstan is inaccessible to any of the processes. I anticipate that
setting the CMDSTAN envvar might help with cmdstanpy too.

TODO (vijay): check that `/opt/cmdstan` install works with cmdstanpy
too.

datascience-docker/Dockerfile

@@ -27,7 +27,12 @@ RUN R --no-restore -e "install.packages(c('remotes'),dependencies=T)"
 RUN R --no-restore -e "remotes::install_github('stan-dev/posteriordb-r')"
 RUN R --no-restore -e "install.packages(c('devtools','mvtnorm','brms','dagitty','fpp3','bsts'),dependencies=T)"
 RUN R --no-restore -e "remotes::install_github('facebook/prophet@*release',subdir='R')"
-RUN R --no-restore -e "remotes::install_github(c('stan-dev/cmdstanr','rmcelreath/rethinking'),dependencies=T);cmdstanr::install_cmdstan()"
+RUN R --no-restore -e "remotes::install_github(c('stan-dev/cmdstanr','rmcelreath/rethinking'),dependencies=T)"
+RUN <<EOT
+mkdir -p /opt/cmdstan
+R --no-restore -e "cmdstanr::install_cmdstan(dir='/opt/cmdstan')"
+EOT
+ENV CMDSTAN="/opt/cmdstan"
 
 RUN <<EOT
 set -ex