Modify Docker image on DockerHub to use AWS Metadata Service for credentials

[acannistra]

The current workflow for building the Docker image that is used in Sagemaker and in Terraform deployments "bakes in" a set of credentials, which breaks for anyone using the image on infrastructure that isn't ours.

Possible avenues for a fix:

• Short Term: build new images for each set of credentials
• Longer Term: investigate whether the EC2 instance service can be accessed from Docker containers + add code to the Docker image that uses the EC2CredentialSource as the source of credentials for the default profile (see: https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-metadata.html)
• Longer Term: investigate whether the SageMaker runtime environment also hosts some sort of credential metadata service similar to the EC2 metadata service for accessing credentials (this way the SageMaker role provides permissions)

cc: @ajijohn thanks for the report about this.

[acannistra]

Confirmed that aws cli within docker container can access EC2 IAM roles.

[acannistra]

Working on this in #25 [045be20]. Removing dependencies on credentials in dockerfile. This likely changes the tutorial contents' dependency on having a configured profile.