[FR] Honouring dockerignore files outside of build context root

[ollie-bell]

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Detailed Feature Information

Which builder is relevant to this request?
docker

Is your feature request related to a problem? Please describe.
Docker docs describes how to name dockerignore files which are not in the build context root. Instead of a ./.dockerignore file you can have <path/to/Dockerfile>.dockerignore alongside <path/to/Dockerfile> (with -f </path/to/Dockerfile> used in the build command). This works locally (MacOS) using Docker version 27.0.3, build 7d4bcd8 on Docker Desktop 4.32.0 (157355). However, in a gcloud build only the ./.dockerignore in the build context root seems to be supported.

Describe the solution you'd like
Support for dockerignore files outside of the build context root as specified in the Docker build documentation
https://docs.docker.com/build/building/context/#filename-and-location
https://docs.docker.com/build/dockerfile/release-notes/#bug-fixes-and-enhancements-18

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.