Chapter 7: Docker Commands, Networks, Volumes and Compose

7.9 Troubleshooting

By Ravindra Bagale Last updated: 25 October 2026

Symptom	Likely cause	What to try
API cannot resolve `db`	Wrong network, or a typo in the service name	Same Compose network; check the service key
Database empty after restart	`down -v`, or an anonymous volume	Named volume; do not pass `-v` by habit
Port already allocated	Host 8080 is nginx or another API	Change the host side to `"8082:8080"` or stop the other container
Auth failed to Postgres	Password mismatch, or an old volume with the old password	Align `.env`, or wipe that volume on purpose
`compose: command not found`	Plugin missing	Install the Compose plugin from Docker's docs
Build context huge	No `.dockerignore`	Same ignore file as Chapter 6

Before the lab, walk the path once. The customer hits host port 8080. The API talks to db:5432 on the private network. Postgres writes the named volume. You restart the stack. The row remains because the volume remained.

Ravi Sir's Cards for this lesson

Troubleshooting Docker. The same few errors cause most Docker pain.

Revision card “Port is already allocated”: what now? Another process or container already uses that host port; find it with ss -tlnp or docker ps, stop it or map a different port.

“Port is already allocated”: what now? Another process or container already uses that host port; find it with ss -tlnp or docker ps, stop it or map a different port.

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