feat: Show auto detected user instructions in 'container test' output

[agatakrajewska]

• Ready for review
• Follows CONTRIBUTING rules
• Reviewed by Snyk internal team

What does this PR do?

Updated 'container test' Dockerfile instructions to "Image layer:" to be consistent with what we show in the UI.
Added showing auto detected instructions if Dockerfile not present.

Where should the reviewer start?

What are the relevant tickets?

https://snyksec.atlassian.net/browse/CAP-149

container test json output

✗ Low severity vulnerability found in bzip2/libbz2-1.0
  Description: Denial of Service (DoS)
  Info: http://localhost:12346/vuln/SNYK-LINUX-BZIP2-106947
  Introduced through: bzip2/libbz2-1.0@1.0.6-8.1, apt/libapt-pkg5.0@1.6.3ubuntu0.1
  From: bzip2/libbz2-1.0@1.0.6-8.1
  From: apt/libapt-pkg5.0@1.6.3ubuntu0.1 > bzip2/libbz2-1.0@1.0.6-8.1
  Image layer: 'RUN test instruction'

[github-actions]

Expected release notes (by @agatakrajewska)

features:
Show auto detected user instructions in 'container test' output (2e3237e)

• I hereby acknowledge these release notes are 🥙 AWESOME 🥙

[agatakrajewska]

Thank you @gitphill !! 😄