feat: variable replacement support for lagoon.base.image label

[bomoko]

This PR extends #355 by adding the ability to specify tags with environment variables.

It allows the lagoon.base.image task to be populated with variables that can be provided to a project or environment and parse them during a lagoon build stage to calculate the required base image to pull.

Variable replacement is similar to envplate. They must be in the ${} format, ie

# no default, will error if `BASE_IMAGE_TAG` is not provided by the project or environment
lagoon.base.image: 'repo/image:${BASE_IMAGE_TAG}' 
# default of `main` if `BASE_IMAGE_TAG` is not provided by the project or environment
lagoon.base.image: 'repo/image:${BASE_IMAGE_TAG:-main}'`
# it is possible to replace more than just the tag too
lagoon.base.image: '${BASE_IMAGE_REPO:-repo}/image:${BASE_IMAGE_TAG:-main}'`

Nested variables like ${BASE_IMAGE_TAG:-${OTHER_IMAGE_TAG:-latest}} will not work.

[shreddedbacon]

After discussing with the team, consensus is that this is fine to merge.