Mongo client connects with command-line params, but fails with equivalent URI

When I create a MongoDB database with this script:

#!/bin/bash javascript=' db.dropUser("admin"); db.createUser({ user: "admin", pwd: "password", roles: [ {role: "userAdminAnyDatabase", db: "admin"} ] }); db.getSiblingDB("example").empty.save({"empty": 0}); ' docker-compose exec mongodb mongo --eval "${javascript}" 

I can successfully login with the mongo shell and command-line parameters:

$ mongo --host docker-machine -u admin -p password MongoDB shell version v3.4.10 connecting to: mongodb://docker-machine:27017/ MongoDB server version: 3.4.10 

But with what seems like the equivalent connection URI, the connection fails:

$ mongo 'mongodb://admin:password@docker-machine/' MongoDB shell version v3.4.10 connecting to: mongodb://admin:password@docker-machine/ 2017-12-06T21:17:47.297-0700 E QUERY [thread1] Error: Authentication failed. : connect@src/mongo/shell/mongo.js:237:13 @(connect):1:6 exception: connect failed 

And the MongoDB log shows:

2017-12-07T04:16:27.703+0000 I ACCESS [conn100] SCRAM-SHA-1 authentication failed for admin on admin from client 10.211.55.2:52496 ; AuthenticationFailed: SCRAM-SHA-1 authentication failed, storedKey mismatch 

What's going on?