Passing environment variables not working with Docker

Question

I am learning how to pass environment variables to Docker containers. While the following works fine,

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Dockerfile

FROM ubuntu
E         


        

Answer 1

OK, I got it. Instead of the following,

docker run -d enviro -e USERNAME='david'

it must be like this

docker run -d -e USERNAME='david' enviro 

No idea, why docker requires the environment variable before the image's name though.