Docker Compose, Registry, and Networking: Multi-Container Setups

The Docker Compose, Docker Registry, and Docker Networking exercises cover multi-container orchestration, image registries, and network drivers. These exercises appear across 4 courses. This article covers common failures and how to fix them.

Table of Contents

1. Docker Compose: Containers Exiting with Code 0
2. Docker Compose File Location Matters
3. Docker Registry: Tagging Before Push
4. Docker Registry: Insecure Registry Error
5. Docker Hub Login and Push
6. Network Drivers: Bridge, Macvlan, None
7. Removing a Network with Active Containers
8. Common Questions

Docker Compose: Containers Exiting with Code 0

No Main Process Means Instant Exit

After docker compose up -d, containers may immediately show Exited (0):

$ docker compose ps -a
NAME            STATUS
myubuntu        Exited (0) 5 seconds ago
myalpine        Exited (0) 5 seconds ago

This happens because the base images (ubuntu, alpine) have no default foreground process.

Fix: Add a process to keep the container running. Options include:

services:
  ubuntu:
    image: ubuntu:24.04
    stdin_open: true
    # or
    command: sleep infinity

The stdin_open: true flag (equivalent to -i) keeps stdin open, preventing the container from exiting.

Docker Compose File Location Matters <a name="compose-location>

Compose Looks for docker-compose.yml in Current Directory

cd ~/ && docker compose up -d

If you run docker compose up from the wrong directory, it won't find the compose file. The exercise typically asks you to create the file at /opt/docker-compose.yml:

docker compose -f /opt/docker-compose.yml up -d

Fix: Always specify -f /path/to/docker-compose.yml or cd to the directory containing the file.

Compose YAML Structure

name: pdso-training
services:
  webserver:
    image: nginx
    container_name: webserver
    ports:
     - 8080:80
    volumes:
     - data:/usr/share/nginx/html
volumes:
  data:

Common mistake: Incorrect indentation under ports and volumes lists. YAML is strict about indentation.

Docker Registry: Tagging Before Push

Image Must Be Tagged with Registry URL

You can't push an image to a registry without first tagging it with the registry address:

# Start local registry:
docker run -d -p 5000:5000 --restart=always --name registry registry:2

# Tag the image with registry URL prefix:
docker tag django.nv:1.0 localhost:5000/django.nv:1.0

# Now push:
docker push localhost:5000/django.nv:1.0

Common mistake: Trying to push django.nv:1.0 directly without the localhost:5000/ prefix. Docker doesn't know which registry to push to.

Verify the Push

curl localhost:5000/v2/_catalog
# Output: {"repositories":["django.nv"]}

Docker Registry: Insecure Registry Error <a name="registry-insecure>

Local Registry Uses HTTP, Not HTTPS

A local registry on localhost:5000 uses plain HTTP. Docker by default refuses to push to non-TLS registries:

http: server gave HTTP response to HTTPS client

Fix: For lab environments, configure Docker to trust the insecure registry by adding to /etc/docker/daemon.json:

{
  "insecure-registries": ["localhost:5000"]
}

Then restart Docker: systemctl restart docker. In the lab environment, this is typically pre-configured.

Docker Hub Login and Push <a name="dockerhub-push>

Login to Docker Hub

docker login
# or
docker login -u your-docker-username -p your-docker-password

The exercise verifies login by checking for /root/.docker/config.json.

Push to Docker Hub Requires Username Prefix

Docker Hub images must be prefixed with your username:

docker tag django.nv:1.0 yourdockerusername/django.nv:1.0
docker push yourdockerusername/django.nv:1.0

Common mistake: Trying to push django.nv:1.0 without the username prefix. Docker Hub rejects pushes to images you don't own.

Network Drivers: Bridge, Macvlan, None

Network Driver Comparison

none Network = No Connectivity

docker run -d --name ubuntu --network=none -it ubuntu:24.04
docker exec ubuntu apt update

Temporary failure resolving 'archive.ubuntu.com'

This is expected behavior — the container has no network stack.

Bridge Network with Custom Subnet

docker network create app --subnet "172.16.0.0/16"

Verify:

docker inspect app | jq '.[].IPAM.Config'

Removing a Network with Active Containers

Can't Remove a Network with Connected Containers

docker network rm app

Error response from daemon: error while removing network: network app id xxx has active endpoints

Fix: Disconnect or remove the container first:

docker network disconnect app myubuntu
# or
docker stop myubuntu && docker network rm app

Key concept: Removing a network doesn't automatically disconnect or stop attached containers.

Connecting a Container to a Network After Creation

docker network connect app myubuntu

Verify:

docker inspect myubuntu | jq '.[].NetworkSettings.Networks.app'

Common Questions

Wrap-Up

Docker Compose, Registry, and Networking exercises cover multi-container orchestration:

• Compose containers need a foreground process — use stdin_open: true or command: sleep infinity.
• Always tag images with the registry URL before pushing (localhost:5000/image:tag).
• Docker Hub requires your username prefix on image names.
• Bridge is the default network driver; none means zero connectivity; macvlan gives containers physical-network-level addresses.
• You can't remove a network while containers are connected to it.

The DevSecOps Box is stateless — all networks, containers, and images are lost on page refresh.