Add note in the documentation about usage of compute credentials to access ECR

[munishchouhan]

From version 1.9.0, the wave also supports retrieval of compute credentials from the platform and uses them to access the container repositories if no credentials for the container registry are provided.
Wave currently support 'aws-batch'

[gwright99]

How does this work if there are multiple AWS creds in a Workspace?

[pditommaso]

It uses the creds associated with the CE

[gwright99]

What happens if the Credential is a User who is supposed to assume a Role via ARN?

A dedicated ECR container credential does not offer the possibility of assuming a Role ARN, whereas most folks derived their CE's AWS permissions from a Role. Is Wave smart enough to do a role assumption with the CE credential?

[pditommaso]

@llewellyn-sl @jason-seqera please coordinate with @munishchouhan in regards this docs task