---
title: "Fix: Docker buildx cache 400 error on CI builds"
description: "Stale buildx_buildkit_builder containers on Gitea Actions runner cause 400 Bad Request when exporting cache to Docker Hub."
type: troubleshooting
domain: development
tags: [troubleshooting, docker, gitea, ci]
---

# Fix: Docker buildx cache 400 error on CI builds

**Date:** 2026-03-23
**Severity:** Medium — blocks CI/CD Docker image builds, requires manual intervention to retrigger

## Problem

Gitea Actions Docker build workflow fails at the "exporting cache to registry" step with:

```
error writing layer blob: failed to copy: unexpected status from PUT request to
https://registry-1.docker.io/v2/.../blobs/uploads/...: 400 Bad request
```

The image never gets pushed to Docker Hub. Seen on both Paper Dynasty and Major Domo repos.

## Root Cause

Stale `buildx_buildkit_builder-*` containers accumulate on the Gitea Actions runner host. Each CI build creates a new buildx builder instance but doesn't always clean up. Over time, these stale builders corrupt the registry cache state, causing Docker Hub to reject cache export PUT requests with 400.

## Fix

Kill all stale buildx builder containers on the runner, then retrigger the build:

```bash
# Kill stale builders
ssh gitea "docker rm -f \$(docker ps -a --format '{{.Names}}' | grep buildx_buildkit_builder)"

# Retrigger by deleting and re-pushing the tag
git push origin :refs/tags/<tag> && git push origin <tag>
```

## Lessons

- Monitor buildx builder container accumulation on the Gitea runner — if more than 2-3 are lingering, clean them up proactively
- Consider adding a cleanup step to the CI workflow that prunes old builders after successful builds
- The `cache-to: type=registry` directive in the workflow is the trigger — without registry caching this wouldn't happen, but removing it would slow builds significantly