Add test for install_shiny_server.sh on rocker/r-bspm:20.04

[eitsupi]

Related to #476

Users may find it easier to install Shiny Server on rocker/r-bspm than to install the R packages in rocker/shiny-verse.

We can expect the script to install Shiny Server to work with rocker/r-bspm:20.04 and so on, so I would like to add a test to confirm that it works.
(Of course, in the future we will need to document which scripts have been tested to work with rocker/r-bspm:20.04 etc.)

I was initially thinking of adding the install_rstudio.sh test too, but realized that this would not work at this time as there is already a non-root user named docker in rocker/r-bspm:20.04.

[eitsupi]

It seems that apt-get update was skipped due to remaining cache and the installation test failed.
I will take a look the upstream Dockerfile later.

rocker-versioned2/scripts/install_shiny_server.sh

Lines 12 to 14 in 77e4b1c

	if [ "$(find /var/lib/apt/lists/* | wc -l)" = "0" ]; then
	apt-get update
	fi