Volumes and Networks

The persistence problem

Containers are ephemeral — delete one and its filesystem goes with it. For databases, uploads, or any state you want to keep, you need a volume.

Two kinds of mounts

• Bind mount — maps a specific path on your host into the container. -v /host/path:/container/path. Great for local dev (live-reloading source code).
• Named volume — Docker manages a storage area for you. -v mydata:/var/lib/postgresql/data. Best for production data — portable and backup-friendly.

# Bind mount — live code in dev
docker run -v $(pwd)/src:/app/src myapp

# Named volume — persistent DB data
docker run -v pgdata:/var/lib/postgresql/data postgres

Ports

Containers have their own network. To reach a service inside one, map a host port to a container port:

docker run -p 8080:80 nginx
#        host:container
#   localhost:8080 -> container port 80

Two containers can both listen on port 80 internally, but each must map to a different host port — you only have one port 8080 on the host.

Networks

• bridge (default) — containers talk to each other and the host via a virtual network.
• host — the container shares the host's network stack (no isolation).
• none — no networking at all.

For multi-container apps, create a custom network so containers can reach each other by name.