Check manifest before attempting to push docker images

[logand22]

This PR updates the publishing of images to Amazon ECR to first check if the image exists before attempting to push to it.

Testing

➜  teleport git:(logan/reentrable-docker-push-amazon-ecr) ✗ export DOCKER_IMAGE_STAGING=146628656107.dkr.ecr.us-west-2.amazonaws.com/gravitational/teleport:10.1.2
➜  teleport git:(logan/reentrable-docker-push-amazon-ecr) ✗ if docker manifest inspect ${DOCKER_IMAGE_STAGING} 2>&1 >/dev/null; then echo "Image already exists"; else echo "Push new image"; fi
no such manifest: 146628656107.dkr.ecr.us-west-2.amazonaws.com/gravitational/teleport:10.1.2
Push new image
➜  teleport git:(logan/reentrable-docker-push-amazon-ecr) ✗ export DOCKER_IMAGE_STAGING=146628656107.dkr.ecr.us-west-2.amazonaws.com/gravitational/teleport:10.1.1
➜  teleport git:(logan/reentrable-docker-push-amazon-ecr) ✗ if docker manifest inspect ${DOCKER_IMAGE_STAGING} 2>&1 >/dev/null; then echo "Image already exists"; else echo "Push new image"; fi
Image already exists

Drone build showing it passes with existing images: https://drone.platform.teleport.sh/gravitational/teleport/14212

[r0mant]

Did you test this on CI?