Fix installed package detection for plumber image

[nuest]

The trestletech/plumber image also sets an ENTRYPOINT, which breaks detection of installed packages:

> containerit::get_installed_packages("trestletech/plumber")
Detected API version '1.40' is above max version '1.39'; downgrading
INFO [2019-11-12 12:29:31] Running command Rscript -e write.csv(x = as.data.frame(installed.packages())[,c("Version")]) in trestletech/plumber
Detected API version '1.40' is above max version '1.39'; downgrading
ARGUMENT 'Rscript' __ignored__

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> pr <- plumber::plumb(commandArgs()[4]); pr$run(host='0.0.0.0', port=8000)
Error in plumber::plumb(commandArgs()[4]) : File does not exist: Rscript
Execution halted

Here are a few attempts to fix that, none of them working yet:

cmd = c("Rscript", "-e", "write.csv(x = as.data.frame(installed.packages())[,c(\"Version\")])")

> client$container$run(image = "trestletech/plumber", cmd = cmd, rm = TRUE, entrypoint = "/bin/bash")
/usr/bin/Rscript: /usr/bin/Rscript: cannot execute binary file
Error: Command 'Rscript -e write.csv(x = as.data.frame(installed.packages())[,c("Version")])' in image 'trestletech/plumber:latest' returned non-zero exit status 126
/usr/bin/Rscript: /usr/bin/Rscript: cannot execute binary file
> client$container$run(image = "trestletech/plumber", cmd = cmd, rm = TRUE, entrypoint = "/bin/sh")
/bin/sh: 0: Can't open Rscript
Error: Command 'Rscript -e write.csv(x = as.data.frame(installed.packages())[,c("Version")])' in image 'trestletech/plumber:latest' returned non-zero exit status 127
/bin/sh: 0: Can't open Rscript
> client$container$run(image = "trestletech/plumber", cmd = cmd, rm = TRUE, entrypoint = "bash")
/usr/bin/Rscript: /usr/bin/Rscript: cannot execute binary file
Error: Command 'Rscript -e write.csv(x = as.data.frame(installed.packages())[,c("Version")])' in image 'trestletech/plumber:latest' returned non-zero exit status 126
/usr/bin/Rscript: /usr/bin/Rscript: cannot execute binary file
> client$container$run(image = "trestletech/plumber", cmd = cmd, rm = TRUE, entrypoint = "/bin/sh")
/bin/sh: 0: Can't open Rscript
Error: Command 'Rscript -e write.csv(x = as.data.frame(installed.packages())[,c("Version")])' in image 'trestletech/plumber:latest' returned non-zero exit status 127
/bin/sh: 0: Can't open Rscript
> client$container$run(image = "trestletech/plumber", cmd = c("Rscript", "-e", "sessionInfo()"), rm = TRUE, entrypoint = "/bin/sh")
/bin/sh: 0: Can't open Rscript
Error: Command 'Rscript -e sessionInfo()' in image 'trestletech/plumber:latest' returned non-zero exit status 127
/bin/sh: 0: Can't open Rscript
> client$container$run(image = "trestletech/plumber", cmd = c("Rscript -e 'sessionInfo()'"), rm = TRUE, entrypoint = "/bin/sh")
/bin/sh: 0: Can't open Rscript -e 'sessionInfo()'
Error: Command 'Rscript -e 'sessionInfo()'' in image 'trestletech/plumber:latest' returned non-zero exit status 127
/bin/sh: 0: Can't open Rscript -e 'sessionInfo()'