Add build_only flag to skip provisioners and avoid /packer-files mount

[krjakbrjak]

This PR introduces a new build_only option to the Docker builder. When enabled, Packer will:

• Only execute the build block (equivalent to running docker build)
• Skip running any provisioners
• Avoid mounting the /packer-files directory into the container

Currently, Packer always mounts a host directory at /packer-files for provisioners. This directory is:

• Unmountable post-build (due to Docker limitations)
• Persisted in the final image, even if empty
• Not needed if provisioners aren’t used

This causes unnecessary artifacts in images and complicates reproducible builds. This issue is described in #134.

Benefits

• Cleaner final Docker images (no /packer-files directory)
• Improved reproducibility: image hashes remain stable for identical manifests
• More predictable builds for CI/CD pipelines using Docker-native workflows

Example usage:

source "docker" "postgres" {
  build {
    path = "Dockerfile"
  }
  build_only = true
  commit     = true
}

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