Build warning for changed cache key

[stuartleeks]

An example build with this occuring is here: https://dev.azure.com/ms/azure-databricks-operator/_build/results?buildId=63849&view=results

The warning is:

The cache task loads data from cache at the start of the build and persists back to the cache after the build. It does this using a cache key, in this case docker-image | .devcontainer/**, from https://github.com/microsoft/azure-databricks-operator/blob/f259cd86d7e823eada878dbbe7bec7205df4b1ce/azure-pipelines.yaml#L30

This means that the key will be docker-image|<hash of the file contents from .devcontainer folder>, implying that the contents of the .devcontainer folder has changed during the build.

Looking at the build, it copies the Makefile into the .devcontainer folder during the build so that it can be used to install kind etc. (from #152)

https://github.com/microsoft/azure-databricks-operator/blob/f259cd86d7e823eada878dbbe7bec7205df4b1ce/azure-pipelines.yaml#L47

If the .devcontainer/Makefile is deleted after the devcontainer has been built then the cache key should match at the end of the build allowing the image to be put into the cache with the same key that is being looked for at the start of the build.