Streamline credentials configuration

[ford-jones]

Note: Depends on #32

Currently, it's required that the user manually set the credentials after the container's creation by using SET or aws_commons.create_aws_credentials.

This pull request makes it much easier to jump into using the tool. Instead of having to set the aws credentials from userspace, they are now set by docker compose. Those functions can now be used to override the default credentials.

Docker compose changes:

• Uses service environment property as opposed to env-file instead of splitting between both
• Sets the s3 request endpoint for usage by the postgres container

Updated docs:

• Recommend plpython3u usage with docker-compose instead of plpythonu
• Adds remarks on setting your own credentials

Cleanup:

• Remove unneeded env file