tools/unitcli: Filter docker images by command

[javorszky]

Closes #1254

Tested by launching a number of containers with the following expectations:

• unit in the name or organisation, but not running unitd as a command ✅ correctly filtered out
• images based on unit:php8.3, but entrypoints and commands customized to check for:
  • nothing before the unitd command (empty entrypoint) ✅ tested locally, works for me
  • nothing after the unitd command (no arguments) ❓ containers exit immediately, can't verify
  • nothing before or after the unitd command ❓ same issue as above, they exit immediately
• container with image name that has no mention of unit (gabor:bla), but running unitd as a command ✅ correctly shows up in instances

[ac000]

Please prefix the commit subjects with tools/unitctl:.

[javorszky]

Please prefix the commit subjects with tools/unitctl:.

done