Add support for `Containerfile` named files for sandbox creation.

Problem Statement

I don't personally use Docker. I don't have it installed on my machines.

I am explicitly passing a file named Containerfile to the sandbox create command, which I would expect to work.

openshell sandbox create --from ./Containerfile

However, I'm greeted with an error:

Error:   × local --from file is not a Dockerfile: ./Containerfile

If I rename this file to Dockerfile it runs without issue.

To be perfectly frank, the --from command shouldn't care what the file is named. I should be able to name it myFile.xyz and so long as it's a valid set of container build instructions it should be able to build the sandbox from this, and if it's not a valid set of build instructions the build tool should spit out the native error as one would expect it to.

Proposed Design

Remove any filename-specific conventions for the --from argument.

Alternatives Considered

Allow Containerfile - while this would solve the immediate issue, it wouldn't allow for any future expansion to other file naming conventions, or it would not allow other custom files to be passed.

Agent Investigation

No response

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request