Source not found on Vagrant up

I have the below code in my Vagrantfile which calls the below script. The script runs fine up until the last line source $dotfile. When it gets to source, the script says source: not found. The line before, cat $dotfile works just fine so the file clearly exists.

Why is this file somehow not found for the source command but it works for the previous cat command?

output error

==> default: /vagrant/scripts/create_functions_dotfile.sh: 14: /vagrant/scripts/create_functions_dotfile.sh: source: not found

Vagrantfile

config.vm.provision "#{script["name"]}", type: "shell" do |shell|
  shell.inline = "/bin/sh /vagrant/scripts/create_functions_dotfile.sh"
end

scripts/create_functions_dotfile.sh

#!/bin/sh

dotfile=/home/vagrant/.functions.sh

for file in /vagrant/scripts/functions/*; do
  echo "cat $file >> $dotfile"
  cat $file >> $dotfile
done

echo "source $dotfile" >> /home/vagrant/.bashrc
cat $dotfile
source $dotfile