IAM_CUSTOM_ENDPOINT and STS_CUSTOM_ENDPOINT env vars not available

[marshall7m]

Within the example Local Stack docker-compose file, custom iam and sts endpoint environment variables are passed as IAM_CUSTOM_ENDPOINT and STS_CUSTOM_ENDPOINT although there are not supported within the image which results in the image to route requests to AWS. Looking at the config.go file within the mainline and v1.4.0 the correct environment variables should be IAM_ENDPOINT and STS_ENDPOINT to enable custom endpoints.