Switch from bash to ruby in docker

#! /bin/bash

# Create a temp file
program=$(mktemp /tmp/XXXX)

# Get the line number of the last match of __PROGRAM_START__
# Just grep-ing the file to find the marker will match the grep itself
cut_pos=$(cat ${0} | grep -n '__PROGRAM_START__' | tail -1 | awk -F ':' '{print $1}')

# Get the docker image tag to run the program in
image_tag=$(tail +${cut_pos} ${0} | head -n 1 | awk '{print $3}')

# Write the program starting from a line after the __PROGRAM_START__ marker,
# and make it executable
tail +${cut_pos} ${0} | tail -n +2 > ${program}
chmod a+x ${program}

# Launch the program in a docker container and exit the bash script
exec docker run --rm -it -v ${program}:${program} ${image_tag} ${program}

# Start of program marker (__PROGRAM_START__), followed by the docker image
# to use for running the program

# __PROGRAM_START__ ruby:2.4
#! /usr/bin/env ruby
puts "hello"