[FR] Honouring dockerignore files outside of build context root

## Notice

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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 `.dockerignore` alongside `` (with `-f ` 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.