Vessel creates different layers for files that haven't been changed

[alan-ghelardi]

Expected Behavior

Subsequent builds run over unchanged files must produce layers with the same SHA256.

Actual Behavior

Vessel produces layers with different SHA256 hashes for files that haven't been changed at all.

Steps to Reproduce the Problem (if it's a bug)

1. Run vessel containerize on a given project and save the resulting tarball as v1.tar.
2. Run vessel containerize on the same project without modifying anything and save the resulting tarball as v2.tar.
3. Inspect the archives and notice that the layers created by Vessel have different hashes for each execution.

Additional Info

• Vessel version: 0.2.128.
• Clojure version: any
• Java version: any

This issue is critical because prevents Vessel from leveraging layers, exactly the purpose that motivated its creation. I need to investigate further, but I guess that the issue has been introduced after this pull request: #4.