How to fix 'docker: invalid reference format.'

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.