Add Containerfile support for dive build command (#536) (#588)

* Add Containerfile support for dive build command

This commit adds Containerfile discovery for `dive build` command for docker engine.
By default, docker uses `Dockerfile` and will fail if the build context contains `Containerfile` instead.
This commit fixes it.

* Add lowercase logic to Containerfile and Dockerfile discovery function

This commit adds:
* Lowercase test for Containerfile discovery function.
  It is now possible to specify `containerfile` or `dockerfile` path
  or discover them in a build context.
* Build arguments `--file` flag test.

* add tests

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Co-authored-by: Alex Goodman <wagoodman@users.noreply.github.com>