As per discussed in #2289

• Update entrypoint.sh to be POSIX compatible (formerly used some BASH only syntax) for eventual switch to an Alpine Image.
• Update entrypoint.sh to include a warning about backwards compatibility and the changes coming to docker images with 3.6.X. Script will automatically change permissions of directories that require update.
• Use eclipse-temurin:21-XXX-noble images for build and run stages of Dockerfile.
• Speed up build time for local builds of Reposilite images by
  • Changing the ordering of Dockerfile instructions to allow for more parallel building and increased cache hits
  • Include an .dockerignore file to exclude most non-application files.