[FEATURE] RUN command with --raw or --raw-output

[alexgwolff]

Description

would like to request the addition of a --raw-output or --raw flag to the RUN command in BuildKit. This flag would output the command result as a raw line, without any BuildKit formatting or prefix, allowing downstream tools (such as CI systems like GitHub Actions) to interpret the output as intended. For example, this would make it possible to emit special commands or error messages that are parsed by CI environments (such as writing errors to a file).

This feature is currently available in Earthly with the --raw-output flag (see: https://docs.earthly.dev/docs/reference/run#output-options). In Earthly, this allows the output to appear without any target name prefix, so CI environments can consume the raw output directly.

Example

Given the following target:

RUN --raw-output echo "::group::"
RUN echo "should have prefix"
RUN --raw-output echo "::endgroup::"

::group::
 #40 0.369 should have prefix from buildkit
::endgroup::

Where the lines with --raw-output appear exactly as emitted by the command, with no prefix or additional formatting.

Motivation

Having this flag in BuildKit would improve integration with CI/CD tools that depend on reading unmodified output from build steps. It would also improve compatibility with scripts and workflows that require emitting special markers or commands (such as GitHub Actions workflow commands) from within a build step.

[AkihiroSuda]

This may bring some vulnerability, as some tools may parse the output of a build script to capture the build result (sha256 digest) and other metrics.
So this feature should be gated by an entitlement like --allow exec.raw-output?

Also, probably escape sequences have to be banned to prohibit faking up the terminal screen