Add support for Amazon ECR

[ejholmes]

How we pass a docker.AuthConfiguration down for pulls should be abstract a bit so that we can generate a docker.AuthConfiguration dynamically to work with ECR, and we can provide various AuthConfigurationProvider implementations:

// Generates a docker.AuthConfiguration from a .dockercfg
FileProvider(file io.Reader) docker.AuthConfiguration {}
// Generates a docker.AuthConfiguration for an Amazon ECR registry.
ECRProvider() docker.AuthConfiguration {}
// Merges multiple docker.AuthConfigurations together.
MultiProvider(providers ...AuthConfigurationProvider) AuthConfigurationProvider

[m87h]

FWIW, I took a stab at this today: wowgroup/empire@14bee7db. I'll open a PR once I've given it another look-over (tomorrow morning by my estimates).

[ejholmes]

@protomouse awesome stuff. If you wanna open a PR, I'd love to get this merged in for the next release.

[m87h]

PR done.

[JacopoDaeli]

@ejholmes @protomouse how can we use ECR with EMPIRE ?

[ejholmes]

#730 was just merged and will be in the next release.

@JacopoDaeli The only thing you should need to do is add the following permissions to your EC2 container instances instance profile:

ecr:GetAuthorizationToken
ecr:BatchCheckLayerAvailability
ecr:GetDownloadUrlForLayer
ecr:BatchGetImage