Why doesn't the python file ( /bin/sh: 1: [python,: not found) read in docker?

Question

Docker

#Dockerfile
FROM python:3.6
RUN mkdir -p /usr/src/app/
WORKDIR /usr/src/app/
COPY . /usr/src/app/

CMD ["python", "app.py"]

#app.py
print('hello world')

error

Answer 1

try CMD ["python3", "app.py"]

Answer 2

CMD has 3 forms:
1. CMD ["executable","param1","param2"] - here you need the full path to the executable
2. CMD ["param1","param2"] - here are the ENTRYPOINT parameters
3. CMD command param1 param2 - this is just a command for the shell
Judging by the error, the third (!) form is used, the shell bites out the quotes and remains:
[python, app.py]
Bold showed the real name of the command that is passed to the shell.
Why is that? But I don't know. Old docker?
My test with such initial data was successful, despite the fact that I do not have a new version (17.05.0-ce).

spoiler

[email protected] ~/dockers/test2 $ docker build -t pythontest .
Sending build context to Docker daemon  3.072kB
Step 1/5 : FROM python:3.6
 ---> 2dfb6d103623
Step 2/5 : RUN mkdir -p /usr/src/app/
 ---> Running in 09f00e2cd7c0
 ---> f7bf180ffc62
Removing intermediate container 09f00e2cd7c0
Step 3/5 : WORKDIR /usr/src/app/
 ---> a35eb6c2b4d2                                                                                                                                                                                                                                                            
Removing intermediate container 28981d13a074                                                                                                                                                                                                                                  
Step 4/5 : COPY . /usr/src/app/                                                                                                                                                                                                                                               
 ---> bf2cafb07143                                                                                                                                                                                                                                                            
Removing intermediate container 0553c72905cb                                                                                                                                                                                                                                  
Step 5/5 : CMD python app.py                                                                                                                                                                                                                                                  
 ---> Running in 0e57967e6d8f                                                                                                                                                                                                                                                 
 ---> 5157a21343e9                                                                                                                                                                                                                                                            
Removing intermediate container 0e57967e6d8f                                                                                                                                                                                                                                  
Successfully built 5157a21343e9                                                                                                                                                                                                                                               
Successfully tagged pythontest:latest                                                                                                                                                                                                                                         
[email protected] ~/dockers/test2 $ docker run -it --rm pythontest                                                                                                                                                                                                                
hello world                                                                                                                                                                                                                                                                   
[email protected] ~/dockers/test2 $

Answer 3

CMD needs double quotes, not single quotes!!
need "" - instead of ''