Copy Docker images in parallel when mirroring to ECR

[achave11-ucsc]

@hannes-ucsc: "We have around ten images to copy. When we used docker pull and docker push to copy them, doing all images in parallel (using Terraforms default concurrency) would overload dockerd/containerd so we added a semaphore to serialize that part. Now that we use skopeo to copy images, we may want to remove the semaphore again or set it to a higher initial value for increased concurrency."

[hannes-ucsc]

Spike to test and measure the running time of deploying tempdev.shared with an initial value of 1, 5 and 10, deleting all images from ECR before every run.

[hannes-ucsc]

Skopeo copies six layers in parallel by default. However, many images have one large layer and many small ones, so copying multiple images in parallel might still be worth it. Hence the spike.