title: cannot install devtools in r shiny docker
body: <p>I'm trying to build a docker image for my shiny app.  Below is my <code>dockerfile</code>. When I build my images, everything else seems fine, except I got error message <code>Error in library(devtools) : there is no package called ‘devtools’ Execution halted</code>.  I also tried <code>devtools::install_github('nik01010/dashboardthemes')</code> with no success.  I have non clue why?  What could go wrong? Do anyone know what is wrong with my dockerfile?  Thanks a lot.</p>
  
  <pre><code># Install R version 3.6
  FROM r-base:3.6.0
  
  # Install Ubuntu packages
  RUN apt-get update &amp;&amp; apt-get install -y \
      sudo \
      gdebi-core \
      pandoc \
      pandoc-citeproc \
      libcurl4-gnutls-dev \
      libcairo2-dev/unstable \
      libxt-dev \
      libssl-dev
  
  # Download and install ShinyServer (latest version)
  RUN wget --no-verbose https://s3.amazonaws.com/rstudio-shiny-server-os-build/ubuntu-12.04/x86_64/VERSION -O "version.txt" &amp;&amp; \
      VERSION=$(cat version.txt)  &amp;&amp; \
      wget --no-verbose "https://s3.amazonaws.com/rstudio-shiny-server-os-build/ubuntu-12.04/x86_64/shiny-server-$VERSION-amd64.deb" -O ss-latest.deb &amp;&amp; \
      gdebi -n ss-latest.deb &amp;&amp; \
      rm -f version.txt ss-latest.deb
  
  # Install R packages that are required
  RUN R -e "install.packages(c('devtools', 'shiny','shinythemes','shinydashboard','shinyWidgets','shinyjs', 'tidyverse', 'dplyr', 'ggplot2','rlang','DT','lubridate', 'plotly',  'leaflet', 'mapview', 'tigris', 'rgdal', 'visNetwork', 'wordcloud2', 'arules'), repos='http://cran.rstudio.com/')"
  RUN R -e "library(devtools)"
  RUN R -e "install_github('nik01010/dashboardthemes')"
  
  
  # Copy configuration files into the Docker image
  COPY shiny-server.conf  /etc/shiny-server/shiny-server.conf
  COPY /app /srv/shiny-server/
  
  # Make the ShinyApp available at port 80
  EXPOSE 80
  
  # Copy further configuration files into the Docker image
  COPY shiny-server.sh /usr/bin/shiny-server.sh
  
  CMD ["/usr/bin/shiny-server.sh"]
  </code></pre>