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In this lesson, we’ll explore advanced Docker flags and best practices for managing containers. You’ll learn how to:
  • Automatically clean up ephemeral containers
  • Customize container hostnames
  • Control container restarts with various policies
  • Inspect Docker events for troubleshooting
  • Copy files between host and containers
  • Expose ports using random and static mappings

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Automatically Remove Ephemeral Containers with --rm

When running short-lived or CI/CD containers, you can use --rm to automatically delete them upon exit. This helps keep your environment clean and free of stale containers. 
docker container run -itd --name=kodekloud --rm ubuntu
docker container ls -l   # Lists only the most recent container
Stop and watch it disappear: 
docker container stop kodekloud
Using --rm is ideal for one-off or CI tasks. Remember that you won’t be able to inspect logs or the filesystem after the container exits.

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Setting Container Name vs. Hostname

Docker allows you to assign both a unique container name and an internal hostname: 
docker container run -itd \
  --name=yogesh \
  --hostname=kodekloud \
  --rm ubuntu

docker container ls -l
docker exec -it yogesh hostname
Container names must be unique per host, but you can reuse the same internal hostname across multiple containers.

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Restart Policies

Docker’s --restart flag offers fine-grained control over container uptime. Below is a summary of each policy:
PolicyDescription
no(default) Container will not restart after exit.
on-failure[:N]Restart only on non-zero exit status. Optionally limit retries to N.
alwaysAlways restart the container regardless of exit code or Docker daemon restarts.
unless-stoppedLike always, but the container won’t restart if you manually stopped it.

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1. --restart=no (Default)

docker container run -itd --name=caseone --restart=no ubuntu
docker container stop caseone
docker container ls -l
The container will remain stopped until you manually start it again.

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2. --restart=on-failure

docker container run -itd --name=casetwo --restart=on-failure ubuntu
docker container kill casetwo   # Simulate a failure
sleep 2
docker container ls -l
The container automatically restarts if it exits with a non-zero status.

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3. --restart=always

docker container run -itd --name=casethree --restart=always ubuntu
docker container stop casethree
docker container ls -l
Regardless of exit reason or daemon restart, the container will be restarted.

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4. --restart=unless-stopped

docker container run -itd --name=casefour --restart=unless-stopped ubuntu
docker container stop casefour
docker container ls -l
Even if Docker restarts, casefour stays down after a manual stop.
Use always or unless-stopped for critical services. Excessive restart loops on failure can degrade performance—consider on-failure with a retry limit.

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Inspecting Docker Events

To diagnose restart loops or networking events, view real-time Docker system events: 
docker system events --since 60m
Sample output: 2020-05-04T08:38:43.747Z network connect cf10… (container=2daf…, name=bridge) 2020-05-04T08:38:43.976Z container start … (image=ubuntu, name=casethree) 2020-05-04T08:39:43.633Z network connect cf10… (container=74b0…, name=bridge) 2020-05-04T08:39:43.890Z container start … (image=ubuntu, name=casefour)

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Copying Files Between Host and Container (docker cp)

You don’t need an interactive shell to move files; docker cp handles it directly.

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Host → Container

mkdir -p /var/temp
echo "hello for KodeKloud" > /var/temp/yogishtest

docker container run -itd --name=copytest --rm ubuntu
docker container exec copytest mkdir -p /root
docker container cp /var/temp/yogishtest copytest:/root/
docker container exec -it copytest cat /root/yogishtest

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Container → Host

docker container exec copytest bash -c "mkdir -p /var/temp && echo 'container to host' > /var/temp/docker-host"
docker container cp copytest:/var/temp/docker-host /var/temp/
ls -l /var/temp/docker-host
cat /var/temp/docker-host

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Publishing Ports

By default, containers don’t expose ports to the host. You can use random or static port mappings:
OptionDescriptionAccess URL
noneNo mapping; container ports are isolatedUnreachable from host
-PRandom high host port mapped to container’s porthttp://<host-ip>:<random>
-pStatic host-to-container port mapping (e.g., 82:80)http://<host-ip>:82

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No Port Mapping

docker container run -itd --name=case1 httpd
docker container ls -l
Accessing via host IP will fail.

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Random Port Mapping (-P)

docker container run -itd -P --name=case2 httpd
docker container ls -l
Docker picks a random port (e.g., 32768) for container port 80.

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Static Port Mapping (-p)

docker container run -itd --name=case3 -p 82:80 httpd
docker container ls -l
Now host port 82 forwards to container port 80.

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Port Mapping on Restart

docker container restart case2
docker container restart case3
docker container ls -l
  • With -P, Docker may assign a different random port after restart.
  • With -p, the mapping remains consistent.
For production services, always use -p for predictable port assignments and easier firewall configuration.

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References

We hope this lesson helps you master advanced Docker container operations!

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