Gitlab CI cache key based on multiple files

.gitlab-ci.yml

build:
  image: node:alpine
  script:
    # Skip build if files are already in cache
    - .gitlab-ci/git-tree-changed.sh .gitlab-cache-key '\.(css|js|json)$' ./frontend ./other-frontend-dir ./public/yet-another || (cd ./frontend && npm ci && npm run build)
  variables:
    # $CACHE_KEY_INDEX is set in project variable, and must be updated each time the CI cache is clear
    # Waiting for the issue to be fixed: https://gitlab.com/gitlab-org/gitlab/-/issues/360438#note_923272903
    CACHE_FALLBACK_KEY: ${CI_DEFAULT_BRANCH}-build-${CACHE_KEY_INDEX}
  cache:
    key: ${CI_COMMIT_REF_SLUG}-build
    paths:
      - .gitlab-cache-key
      - frontend/node_modules
      - frontend/build

git-tree-changed.sh

#!/usr/bin/env sh
set -euo pipefail

# Detect changes in a git tree to generate a cache key
# Hash of the updated files is stored into a file that must be keep in cache

# Install git (for alpine). Should be in the machine for perfs.
command -v git > /dev/null || (apk update && apk add git)

if [ "$3" == "" ]
then
  echo "Missing arguments."
  echo "Exemple:"
  echo "    git-tree-changed.sh .cache-git-hash '\.(js|css|json)$' ./"
  exit 255
fi

cache_file="$1"
grep_pattern="$2"

# Compute a hash from the hash of all versioned files, filtered.
treehash=$(for arg; do git ls-tree -r HEAD --full-name $arg; done|grep -E $grep_pattern|md5sum|cut -d' ' -f1)

if [ ! -f "$cache_file" ]
then
  echo "No cache"
  echo "$treehash" > "$cache_file"
  exit 2
fi

cachedhash=$(cat "$cache_file")

if [ "$cachedhash" == "$treehash" ]
then
  echo "No change detected"
  exit 0
else
  echo "Change detected"
  echo "$treehash" > "$cache_file"
  exit 3
fi