Summary
The issue arises when attempting to set index level annotations in Github Actions for container images using the docker build command with the --annotation flag. Despite the command working locally, Github Actions returns an error stating “index annotations not supported for single platform export”. This problem occurs when using tools like kargo that require annotations at the index level, not the manifest level.
Root Cause
The root cause of this issue is the difference in how Docker handles build commands in local environments versus in Github Actions. When running docker build locally, Docker can handle index-level annotations without issues. However, in Github Actions, the environment and configurations are optimized for single-platform exports by default, which does not support index-level annotations.
Why This Happens in Real Systems
This discrepancy happens because real-world systems, especially those leveraging Github Actions for automated workflows, often rely on optimized and standardized environments. These environments are tuned for performance and compatibility across a wide range of use cases, which sometimes means they don’t support all features or flags available in local development environments. The limitation on index annotations in single-platform exports is one such case.
Real-World Impact
The real-world impact is significant for teams relying on tools like kargo for container image management and analysis. The inability to set index-level annotations in Github Actions hinders the automation of build, promotion, and deployment processes. This can lead to manual workarounds, increased error rates, and slower deployment cycles.
Example or Code
docker build. --push --annotation "index:org.opencontainers.image.source=www.example.com" --annotation "index:org.opencontainers.image.revision=v42" -t ghcr.io/repo/image:tagThis example command works locally but fails in Github Actions with the error “index annotations not supported for single platform export”.
How Senior Engineers Fix It
Senior engineers fix this issue by understanding the limitations of Github Actions and the requirements of the tools they’re using, such as kargo. They might use workarounds like setting BUILDKIT_MULTI_PLATFORM to enable multi-platform builds, even if the project doesn’t explicitly require them, to bypass the single-platform export limitation. Another approach is to modify the CI/CD pipeline to handle annotations in a way that’s compatible with both local development and Github Actions environments.
Why Juniors Miss It
Junior engineers might miss this issue because they are less familiar with the nuances of Docker, Github Actions, and the specific requirements of tools like kargo. They might not fully understand how different environments (local vs. Github Actions) handle Docker build commands and annotations, leading to frustration when commands that work locally fail in automated workflows. Additionally, the error message does not directly point to the root cause, requiring additional research and experience to resolve.