posix compliant script to upgrade all subfolder docker compose services

upgrade.sh

#!/bin/bash
set -ex
if [ -z "$1" ]; then
  for dir in */; do [ -e "$dir" ] || continue; ./upgrade.sh "$dir"; done
  exit 0
fi
cwd="$(pwd)"
cd "$1"
  if [ -f Dockerfile ]; then
    image=$(cat Dockerfile | awk 'tolower($1) == "from" { print $2 }')
  elif [ -f docker-compose.y*ml ]; then
    image=$(cat docker-compose.y*ml | awk 'tolower($1) == "image:" { print $2 }')
  else
    echo "Subfolder $1 must contain a docker-compose.y*ml"
    exit 1
  fi
docker pull $(echo $image) | grep -i 'image is up to date' && exit 0
docker-compose down
docker-compose up -d
docker system prune -f
cd "$cwd"