How to use variables converted to lowercase in Bamboo plan tasks?

[P M]

Is there a way to convert a defined build-specific Bamboo variable to lowercase in plan tasks?

Example:

instead of

${bamboo.planRepository.branch}

I could use something like:

lowercase(${bamboo.planRepository.branch})
lc(${bamboo.planRepository.branch})
${bamboo.planRepository.branch.toLowerCase}

Background: in my repo before a pull request can be merged an automated build executed in Bamboo needs to be successful. I'm using docker task in which image tag contains Git repository branch name, but docker tags must be lowercase while Git branch names - do not have to (developers name their feature branches as they want). This triggers an error in the build (not related with branch code quality)...