wslc build: support tar build contexts from stdin

Is your feature request related to a problem? Please describe.

wslc build currently requires its positional build context to be a directory. It does not support using - to read a tar build context from standard input:

tar -czf context.tar.gz -C context .
wslc build --tag stdin-context:test - < context.tar.gz

Currently, - is interpreted as a host filesystem path and rejected because it is not a directory.

This prevents tools and pipelines from generating or filtering a build context as a tar stream and passing it directly to WSLC. Callers must instead materialize the context as a temporary directory, adding disk I/O, cleanup, and failure-handling requirements.

This is distinct from --file -, which reads only the Dockerfile from stdin while the build context still comes from a directory.

Describe the solution you'd like

Allow - as the positional build context:

wslc build [OPTIONS] -

When stdin contains a tar archive, use the archive contents as the filesystem build context. The default Dockerfile and .dockerignore should be resolved from the archive root, and --file <path> should resolve the specified Dockerfile relative to that root.

At minimum, uncompressed tar archives should be supported. Ideally, this would match Docker's support for identity, gzip, bzip2, and xz-compressed tar streams.

Invalid archives or an interactive stdin should produce an actionable error. If both the context and --file are -, the command should also report clearly that both inputs cannot consume the same stream.

This would provide behavior equivalent to:

docker build - < context.tar.gz
docker build --file test.Dockerfile - < context.tar.gz

Describe alternatives you've considered

• Unpack the tar stream into a temporary directory and pass that directory to wslc build. This works, but defeats streaming and requires additional disk space, cleanup, and error handling.
• Use --file -. This only supplies the Dockerfile through stdin and does not provide the files needed by COPY, ADD, or other context-dependent operations.

Additional context

Docker documents tar build contexts supplied over stdin here:

https://docs.docker.com/build/concepts/context/#local-tarballs