Support .in Dockerfiles via cpp when available#1230
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Support .in Dockerfiles in Dev Containers
Issue Description:
Podman supports building from Dockerfile.in files by using the system cpp preprocessor to generate a valid Dockerfile before the build runs.
Dev Containers do not currently support this workflow. The existing CLI reads the Dockerfile directly, identifies the final FROM stage, and then extends it with features.
When the input is a Dockerfile.in that is not preprocessed, the CLI did not see a resolved FROM instruction, and the build fails with error
Error parsing Dockerfile: Dockerfile contains no FROM instructionsSolution:
If cpp is available in the execution environment, the CLI uses it to generate a resolved Dockerfile, allowing Dev Containers to support .in files for both Docker and Podman. If cpp is unavailable, the CLI should fail with a clear error message