Support AWS IAM Roles for Service Accounts

[jordanjennings]

dockcmd currently has a hard-coded list of credentials providers which doesn't support the WebIdentityRoleProvider needed for IRSA to work as expected.

https://github.com/boxboat/dockcmd/blob/master/cmd/aws/aws.go#L126

var creds = sess.Config.Credentials
	if o.useChainCredentials {
		creds = credentials.NewChainCredentials(
			[]credentials.Provider{
				&credentials.EnvProvider{},
				&credentials.SharedCredentialsProvider{
					Profile: o.profile,
				},
				&ec2rolecreds.EC2RoleProvider{
					Client: ec2metadata.New(sess),
				},
				&SessionProvider{
					Session: sess,
				},
			})
	} else {
		if o.accessKeyID == "" || o.secretAccessKey == "" {
			return nil, errors.New("no aws credentials provided")
		}
		creds = credentials.NewStaticCredentials(o.accessKeyID, o.secretAccessKey, "")
	}

I think the entire true section of the block that re-configures the chain of credentials providers can be removed so that it uses the SDK default chain, which already supports IRSA. I'm assuming there was a reason that the custom chain of credentials providers was originally added, but it's not clear what that reason is.

@boxboatmatt Can the custom credentials chain be removed when useChainCredentials is true?