Document Pull from Amazon ECR

[tri-adam]

Amazon Elastic Container Registry (ECR) is a private Docker registry that works out of the box with Singularity. But using it is a bit weird. We should document the process to use it with Singularity 3.x.

Essentially, it amounts to:

$ AWS_PROFILE=blah aws ecr get-login
docker login -u AWS -p <redacted> https://123456789.dkr.ecr.us-east-1.amazonaws.com
$ export SINGULARITY_DOCKER_USERNAME=...
$ export SINGULARITY_DOCKER_PASSWORD=...
$ singularity pull docker://123456789.dkr.ecr.us-east-1.amazonaws.com/image:tag