Also, for the benefit of future readers of this thread, there is an error in the docs on how the lambda-url superflag is set via the environment section of a docker compose file. See Docs(Lambda): Fix incorrect environment variable example by matthewmcneely · Pull Request #449 · dgraph-io/dgraph-docs · GitHub
TL;DR:
DGRAPH_ALPHA_GRAPHQL: lambda-url=http://lambda:8686/graphql-worker
instead of DGRAPH_ALPHA_GRAPHQL_LAMBDA_URL