Need single AND double quotes on CRAN repo URL to protect the backslashes

Dockerfile

@@ -8,7 +8,7 @@ MAINTAINER Mark McCahill "mark.mccahill@duke.edu"
 ENV DEBIAN_FRONTEND noninteractive
 ENV R_VERSION="3.6.3"
 ENV RSTUDIO_VERSION="1.2.1335"
-ENV CRAN_REPO='https://mran.revolutionanalytics.com/snapshot/2020-04-23'
+ENV CRAN_REPO="'https://mran.revolutionanalytics.com/snapshot/2020-04-23'"
 
 # get R from a CRAN archive 
 RUN apt-get update && \