chore: refactor resource dockerfile

[dduportal]

While working on #390 and #412, the need to refactor the dockerfile resource was raised.

This PR does not change the functionnality of the resource: it's why the label says "chore", except for the resulting commit messages (that were not really correct).

It introduces the following changes:

• Fix the example to allow end to end testing from the root of the repository
• Apply the same conventions as other refactored resources:
  • Define a Spec type on the package with only the public attributes
  • Add a New initializer method
• Improve Developer experience:
  • Full unit test with mocked injected dependencies (no more file to be read/written on the FS while running tests!)
  • Increased test coverage that helped cleaning up unused struct attributes

Test

To test this pull request, you can run the following commands:

go build -o ./dist/updatecli && ./dist/updatecli diff --config examples/updatecli.generic/dockerfile

Additional Information

Tradeoff

Potential improvement

[olblak]

It looks good to me
🚢