Investigate if the build cache for the docker image is used in exalsct

[tkilias]

Background

• In (Add option to) Re-use images from a previous build run script-languages-release#245 was asked if we keep the docker build cache and if we can use it
• When docker builds images each step in a Dockerfile produces an intermediate image which is kept in the build cache
• Because, exaslct copies the build context to a temporary directory, we probably can't reuse these intermediate images, because Docker doesn't associate the new directory with a previous build
• As such, keeping the build cache is likely not useful

Tasks

• Investigate if the docker build cache can be used?
• If it can't be used, check if we keep it.
• If we keep it, change that.

Acceptance Criteria

• Either, the build cache is used and improve the performance of rebuilds of a single images, or we remove the build cache after the build