Add AWS_REGION as a default env as a fallback

Fixes: #4 (closed)

By setting a default ENV in the Dockerfile we can create a fallback for the AWS_REGION trouble.

Functional test

Clone, pull and checkout this branch. Build the project locally:

TRAEFIK_VERSION=v2.10 docker build -t traefik_2_9 -f Dockerfile .

After a succesful build, run the image:

 docker run -ti traefik_2_9 /bin/sh -c 'export'

• You should see export AWS_REGION='eu-central-1'