refactor: move all shellcheck disable definitions to .shellcheckrc file

This makes standard use of shellcheck work without needing to provide extra configuration anywhere.

Also remove use of inline 'shellcheck disable' calls that are already defined in the .shellcheckrc and don't need to be set inline.

bootstrap.sh

@@ -220,19 +220,15 @@ install-dokku-from-deb-package() {
   [[ -n $DOKKU_NGINX_ENABLE ]] && echo "dokku dokku/nginx_enable string $DOKKU_NGINX_ENABLE" | sudo debconf-set-selections
 
   if [[ -n $DOKKU_CHECKOUT ]]; then
-    # shellcheck disable=SC2086
     apt-get -qq -y $NO_INSTALL_RECOMMENDS install "dokku=$DOKKU_CHECKOUT"
   else
-    # shellcheck disable=SC2086
     apt-get -qq -y $NO_INSTALL_RECOMMENDS install dokku
   fi
 }
 
 main() {
   export DOKKU_DISTRO DOKKU_DISTRO_VERSION
-  # shellcheck disable=SC1091
   DOKKU_DISTRO=$(. /etc/os-release && echo "$ID")
-  # shellcheck disable=SC1091
   DOKKU_DISTRO_VERSION=$(. /etc/os-release && echo "$VERSION_ID")
 
   export DEBIAN_FRONTEND=noninteractive