Docker

Problem:

You want to start docker container but you see an error message like this:

docker: invalid reference format.

Solution:

Docker is telling you that the syntax of the docker image name (& version) is wrong. Note that this is not the same as docker not being able to find the image in the registry. Docker will not even be able to lookup the image in the registry if you see an invalid reference format error!

Common causes include:

• You used a colon at the end of the image name, e.g. ubuntu: – omit the colon ; using just ubuntu will refer to ubuntu:latest
• You used a dash at the end of the image name, e.g. ubuntu- – omit the dash ; using just ubuntu will refer to ubuntu:latest
• You used variables like ubuntu:$VERSION but $VERSION is not set. Ensure you have set $VERSION to an appropriate (non-empty) value like latest or 18.04!
• You used multiple colons like ubuntu:18.04:2 or ubuntu:ubuntu:18.04. Use only a single colon in the image name!
• You mixed up the order of command line arguments, so another argument is being interpreted as the image name. Check the syntax of your docker command, e.g. docker run and compare some examples with your command.

If you want to interactively edit a file in a docker container, you might want to install an editor like GNU nano (for example to debug your config files) in your Docker container that allows you direct access to the container’s file system.

docker exec -it [container name or ID] bash -c 'apt-get -y update && apt -y install nano'

This will work for most debian/Ubuntu-based containers ; for other containers you might need to use a package manager other than apt

Now you can use docker exec -it to interactively edit a config file, e.g.:

docker exec -it [container name or ID] nano /etc/host.conf

docker exec runs a program in a container that’s already running. For example, you can use it to create a backup in a container that currently runs your database server.

docker run starts a new container and runs a command in it. For example, your can use it to run a specific script in a container.

Note: This will only remove docker images without a tag, but not all images not associated to a running or stopped container. See our post on How to automatically cleanup (prune) docker images daily in case this is not the desired behaviour.

docker image prune provides an easy way to remove “unused” (i.e. untagged) docker images from a system and hence fixes or significantly delays docker eating up all your disk space on.

I created a systemd-timer based daily prune routine using TechOverflow’s Simple systemd timer generator.

Quick install using

wget -qO- https://techoverflow.net/scripts/install-cleanup-docker.sh | sudo bash

This is the script which automatically creates & installs both systemd config files.

#!/bin/sh
# This script installs automated docker cleanup via "docker image prune"
# onto systemd-based systems.
# It requires that docker is installed properly

cat >/etc/systemd/system/PruneDocker.service <<EOF
[Unit]
Description=PruneDocker

[Service]
Type=oneshot
ExecStart=/usr/bin/docker image prune -f
WorkingDirectory=/tmp
EOF

cat >/etc/systemd/system/PruneDocker.timer <<EOF
[Unit]
Description=PruneDocker

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target
EOF

# Enable and start service
systemctl enable PruneDocker.timer && sudo systemctl start PruneDocker.timer

To view logs, use

journalctl -xfu PruneDocker.service

To view the status, use

sudo systemctl status PruneDocker.timer

To immediately cleanup your docker images, use

sudo systemctl start PruneDocker.service

Use

docker image prune -f

to remove all unused/old (“dangling”) docker images. -f will skip the confirmation prompt.

This command will remove all images that a) don’t have a tag themselves and b) no image with a tag depends on the image.

Example output:

Deleted Images:
deleted: sha256:b6e0e69c6ba1e811063921d18b52627a9fb905e0bdfd80e226e86958831df636
[...]
deleted: sha256:6a40d04ddf00ad0f1806df7b2d4a2d44a6d8031cab5c369a4bf3d1694d5c48b4

Total reclaimed space: 985.3MB

Problem:

When running docker-compose start, you see an error message like this:

Starting mongodb     ... error
Starting myapp       ... error

ERROR: No containers to start

Solution:

In order to start your containers, use docker-compose up, instead of docker-compose start!

Start all services

docker-compose up -d

Stop all services

docker-compose down

docker-compose restart

docker-compose pull
docker-compose restart

View logs

docker-compose logs

To view and follow use

docker-compose logs -f

Start a specific service (and all the services it depends on)

docker-compose start myservice

Show info about which container images are being used

docker-compose images

Run this command in your shell:

docker rm $(docker ps -a -q)

Often, when installing deb packages in your Dockerfile, some packages will install tzdata as a dependency.

The tzdata installer will try to interactively prompt you for your location using

Configuring tzdata
------------------

Please select the geographic area in which you live. Subsequent configuration
questions will narrow this down by presenting a list of cities, representing
the time zones in which they are located.

 1. Africa      4. Australia  7. Atlantic  10. Pacific  13. Etc
 2. America     5. Arctic     8. Europe    11. SystemV
 3. Antarctica  6. Asia       9. Indian    12. US
Geographic area:

This will stall your image build.

In order to fix that, we’ll need to make the tzdata prompt non-interactive.

The preferred method is to add

ENV DEBIAN_FRONTEND=noninteractive

before the first RUN statements in your Dockerfile.

Alternatively you can run just the apt install or apt-get install command using DEBIAN_FRONTEND=noninteractive:

RUN DEBIAN_FRONTEND=noninteractive apt install -y tzdata

This will automatically select a default configuration for tzdata.

Problem:

You want to delete a docker image using a command like

docker image rm c91b419ac445

but you see an error message like

Error response from daemon: conflict: unable to delete c91b419ac445 (cannot be forced) - image is being used by running container 3477a4dcdce2

Solution:

There is currently a container running that uses the image you are trying to delete. We will solve this issue by first stopping the container and then deleting the image

Warning: Deleting the image is dangerous since you cannot undo deleting the image ! Also note that force-stopping a running container might result in data loss if that container is doing something important !

Run these commands to stop the container and delete the image:

# Force-stop the container
docker container rm --force <container ID>
# Delete the image
docker image rm <image ID>

Copy <container ID> from the end of your original error message (3477a4dcdce2 in my example).

Copy <image ID> from the beginning of your error message. This is the same image ID you originally intended to delete (c91b419ac445) in my example.

In my example, the command would be

# Force-stop the container
docker container rm --force 3477a4dcdce2
# Delete the image
docker image rm c91b419ac445

Note that there might be multiple containers running using this image, so if you keep getting a similar error message, you might need to repeat this command.

Background information:

Docker will not allow you to force-delete the image using

docker image rm c91b419ac445 --force

as you can also see from the (cannot be forced) clause of your original error message. This behaviour makes sense since the container would crash in an undefineable manner if the underlying image is deleted.

Note that we could use docker image rm --force after stopping the container but this is typically not required and might result in additional risks for other containers, e.g. if other images depend on said image. Since docker uses layered images. Read the background information section of our post Docker: Remove all images and containers to learn more about how docker images work from an image management perspective.

Problem:

You want to push an image to Dockerhub using a command like

docker push myusernameondocker/my-image

but you see an error message like

denied: requested access to the resource is denied

Solution:

First ensure that your local docker client is logged in to Docker by using

docker login

This will ask you for your username and password. In case you have not registered yet on Dockerhub, register here!

Now you can retry your command. Note that you do not need to create the repository on DockerHub explicitly, it will be created automatically!