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 ENV USERNAME='david' CMD echo "username = $USERNAME" 

Build & run commands

docker build . -t enviro docker run -d enviro 

docker ps -a gives

2a3a69aa7868        enviro              "/bin/sh -c 'echo \"u…" 

docker logs 2a3a69aa7868 gives

username = david 

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The following doesn't work

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Dockerfile

FROM ubuntu CMD echo "username = $USERNAME" 

Build & run commands

docker build . -t enviro docker run -d enviro -e USERNAME='david' 

Here the run command gives this,

docker: Error response from daemon: OCI runtime create failed:    container_linux.go:348: starting container process caused "exec: \"-e\":    executable file not found in $PATH": unknown. 

While docker ps -a gives

249cb045c26a        enviro              "-e USERNAME=david" 

docker logs 249cb045c26a gives nothing

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Any idea, what is going on here? Why is the environment variable not being passed?

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.