`main` branch containers should also be `latest`

[cooperj]

In the .github/workflows/ folder we have some workflows to build docker images.

The docker images create tags in the format of version-branch. Where branch is either the git branch, tagged release number.

What is missing is the latest varient. So for example we are not making lcas.lincoln.ac.uk/ros:humble-latest. We have made that image previously and should create it.