Question 1

Why can my Docker container not connect to a service on localhost?

Accepted Answer

Inside a container, localhost means the container itself, not the machine running it. Each container has its own network namespace and its own loopback interface, so a connection to localhost from inside reaches only processes in that same container. That is why a database running happily on your machine is unreachable and the error is a flat connection refusal.

What to use instead depends on where the other service actually is.

- Another container in the same compose project. Use the service name as the hostname. Compose puts them on a shared network with DNS entries for each service, so the name resolves to that container address. Use the port the service listens on inside the container, not the port published to your machine, because you are not going via the host at all.
- A service on the host machine. Docker provides a special hostname for this on desktop platforms which resolves to the host from inside a container. On Linux it is available in recent versions but may need declaring explicitly as an extra host entry.
- A container on a different network. Containers can only reach each other by name if they share a user-defined network, so attaching both to the same network is the fix.

A closely related mistake sits on the listening side. A service bound to the loopback interface inside its container accepts connections only from that container, no matter how ports are published. Binding to all interfaces is required for anything expected to receive outside traffic.

When debugging, opening a shell inside the container and attempting the connection from there is far more informative than testing from the host, because it exercises the same namespace and DNS resolution your application actually uses.

Question 2

Why does my Node process fail with EADDRINUSE address already in use?

Accepted Answer

The error means something is already listening on the port your process tried to bind. The operating system allows only one listener per port and address combination, so the second attempt is

Question 3

What is the difference between a Docker volume and a bind mount?

Accepted Answer

Both make data survive a container being removed, and they differ in who manages the storage. A bind mount maps a specific path on the host into the container. You control exactly where the data

Question 4

Why does my Docker container exit immediately after starting?

Accepted Answer

A container lives exactly as long as its main process. When the process started by the image's CMD or ENTRYPOINT finishes, the container stops, regardless of anything else running inside it. So an

Question 5

Why does my API return 502 Bad Gateway?

Accepted Answer

A 502 is reported by a proxy or load balancer, not by your application. It means the intermediary tried to forward the request to an upstream server and received an invalid response, or none at all.

Question 6

What is the difference between a Docker image and a container?

Accepted Answer

An image is a static, read-only template. A container is a running, or stopped, instance created from one. The relationship is close to that between a program on disk and a process running it: one

Question 7

How can I troubleshoot MySQL connection issues in a Python application?

Accepted Answer

To troubleshoot MySQL connection issues in Python, check credentials, verify host and port, inspect server status, and review error messages.

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