Fix .dockerignore for build context copies in later stages

[tscni]

Description

.dockerignore currently isn't being applied to build context copies during later (non-first) stages.
E.g.

FROM scratch AS a
COPY a .

FROM scratch AS b
COPY b . # <-- .dockerignore doesn't apply here

This PR adjusts the .dockerignore integration test to surface the issue and fixes it by refactoring the excluded file handling.
Instead of setting more global state, this replaces it with a FileContext struct (containing a root and excluded files), and passes that from the build start down to all relevant commands / functions.
A COPY --from then simply overwrites the FileContext without any excluded files. The handling around IsFirstStage is no longer necessary.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
  Doesn't seem necessary.
• Adds integration tests if needed.
  Well, a modification really.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Fix .dockerignore for build context copies in later stages