removed empty continuation lines in Dockerfile template

[nevrome]

I came across a warning when I tried to reproduce #89/#90 in docker version 18.09.8-ce, build 0dd43dd87f

[WARNING]: Empty continuation line found in:
    RUN . /etc/environment   && sudo apt-get update   && sudo apt-get install libudunits2-dev -y   && R -e "devtools::install('/<REPO>', dep=TRUE)"   && R -e "rmarkdown::render('/<REPO>/analysis/paper/paper.Rmd')"
[WARNING]: Empty continuation lines will become errors in a future release.

Removing the empty lines should fix it.

Steps to reproduce: (stolen from @ntrlshrp)

• In R, devtools::install_github("benmarwick/rrtools")
• rrtools::create_compendium("rrtoolsTEST")
• Switch to new project/package rrtoolsTEST
• rrtools::use_dockerfile()
• Change rocker/verse:### to rocker/verse:3.5.3 in rrtoolsTEST/Dockerfile line 1
• Switch to Terminal where current directory is rrtoolsTEST/
• docker build --rm -t rrtoolstest:1.0 . (you may need to prepend sudo)

[benmarwick]

Thank you!