What This Error Means
Docker reached the registry, but the registry returned HTTP 503 Service Unavailable, usually due to outage, overload, or proxy issues.
How to Fix It
Retry with debug logs:docker --debug pull <image>
Validate DNS and connectivity to the registry host from the same machine.
If you use a proxy, ensure Docker is configured to use it (daemon + CLI env).
If the registry is internal, confirm health and TLS cert validity.
Why It Happens
DNS, firewall, proxy, or VPN is blocking registry traffic.
The registry is temporarily unavailable or overloaded.
Corporate proxies are interfering with TLS or long-lived connections.
How to Verify
Re-run the original pull/push and confirm it completes.
Confirm CI runners can reach the same registry endpoint.
Examples
Error response from daemon: Get https://registry-1.docker.io/v2/: Service Unavailable
Error response from daemon: received unexpected HTTP status: 503 Service UnavailablePrevention Tips
Use a proxy/cache registry to reduce dependence on upstream availability.
Keep runner network configuration consistent (DNS/proxy/firewall).