Updates the docker-compose.yml to 3.8

[lezavala]

Issue #, if available:
#20

Description of changes:

• Updates the docker-compose.yml to 3.8
• Removes gateway key in ipam config, seems not supported
• Uses alias to generate the ECS_CONTAINER_METADATA_URI environment variable and removes static IPs which do not seem to work

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[BenHarris]

Seems much cleaner than the old approach, but it will not work with roles as 169.254.170.2 is hardcoded into the EcsCredentialProvider

[PettitWesley]

@BenHarris is correct, that special IP address is a requirement

[BenHarris]

Would implementing an AWS_CONTAINER_CREDENTIALS_SERVER_URI environment variable in the EcsCredentialProvider to facilitate overriding the SERVER_URI be a possibility? I appreciate this might not be the best place to raise the issue, but it would be nice to not have to assign a static IP to each container.

EDIT: I've just tested the above locally and it works well. It has the added benefit that the credentials_network isn't required at all, which takes a massive amount of junk out of your docker-compose file if you aren't already using custom networking.