Redirecting command output in docker

When you specify a JSON list as CMD in a Dockerfile, it will not be executed in a shell, so the usual shell functions, like stdout and stderr redirection, won't work.

From the documentation:

The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

What your command actually does is executing your index.py script and passing the strings "1>server.log" and "2>server.log" as command-line arguments into that python script.

Use one of the following instead (both should work):

1. CMD "python index.py > server.log 2>&1"
2. CMD ["/bin/sh", "-c", "python index.py > server.log 2>&1"]

To use docker run in a shell pipeline or under shell redirection, making run accept stdin and output to stdout and stderr appropriately, use this incantation:

docker run -i --log-driver=none -a stdin -a stdout -a stderr ...

e.g. to run the alpine image and execute the UNIX command cat in the contained environment:

echo "This was piped into docker" |
  docker run -i --log-driver=none -a stdin -a stdout -a stderr \
    alpine cat - |
  xargs echo This is coming out of docker: 

emits:

This is coming out of docker: This was piped into docker

Just a complement, when using docker-compose, you could also try:

command: bash -c "script_or_command > /path/to/log/command.log 2>&1"