#745 environment variables to be used instead of 'region' property for aws-ecr registry

[quintok]

I thought I'd give this a go as it looked like a pretty simple change and a good "first issue"

A few thoughts:

1. This logic should be centralised for all aws environment config.
2. Packer and terraform use https://github.com/hashicorp/aws-sdk-go-base which seems reasonable to migrate to.
3. This logic is too simple, it should try and lean on the SDK more (ie: load region from default profile)
4. The logic about leaning on the sdk seems similar between terraform and packer - maybe we can push that into the aws-sdk-go-base?

Finally, my dev environment is not healthy enough to check the documentation output.
After installing protobuf from homebrew and running:
make gen/doc

./vendor/proto/api-common-protos/: warning: directory does not exist.
google/rpc/status.proto: File not found.
internal/server/proto/server.proto:10:1: Import "google/rpc/status.proto" was not found or had errors.
...

[hashicorp-cla]

All committers have signed the CLA.

[mitchellh]

I’ll note that I do want to use the AWS SDK base lib we made but there was a reason we didn’t for 0.1 that I can’t recall. That would definitely be the more correct longer term solution.

[briancain]

Thanks for this! Looks good to me 👍 We'll try to use the aws sdk in the future, but for now this looks good in the short term.