Fix helper docker image tar output
requested to merge 26618-could-not-lock-config-file-root-gitconfig-operation-not-permitted-error-in-docker-executor into master
What does this MR do?
Returns to using docker export to save the tar versions of the helper docker container.
Why was this MR needed?
#26618 highlighted a case where the method of exporting tar matters if the docker filesystem driver is aufs.
What's the best way to test this MR?
make runner-and-helper-docker-host - test the resulting images.
Testing the images with a aufs filesystem may prove more difficult. I had to boot a Ubuntu image on GCP, and change the docker filesystem to aufs to test.
What are the relevant issue numbers?
Closes #26618