using ENTRYPOINT ["/sbin/my_init"] runs differently than CMD ["/sbin/my_init"]?

[Michael Dalpee]

I have a Docker image that uses phusion/baseimage.  In my dockerfile, if I use ENTRYPOINT ["/sbin/my_init"] and run the final image using:

 docker run --rm -t -i my_container

It works as expected and I get the following ouput:

*** Running /etc/my_init.d/00_regen_ssh_host_keys.sh...

*** Running /etc/rc.local...

Starting the Web Server

*** Booting runit daemon...

*** Runit started as PID 10

Spawning Xvfb

Allow anyone to connect to Xvfb

access control disabled, clients can connect from any host

=> Booting Puma

=> Rails 5.1.4 application starting in production 

=> Run `rails server -h` for more startup options

Capybara.threadsafe == true is a BETA feature and may change in future minor versions

Overwriting existing field _id in class ScrapeJob.

Overwriting existing field _id in class ScrapeJobStatus.

Overwriting existing field _id in class ScrapeSet.

[9] Puma starting in cluster mode...

[9] * Version 3.11.0 (ruby 2.4.2-p198), codename: Love Song

[9] * Min threads: 5, max threads: 5

[9] * Environment: production

[9] * Process workers: 2

[9] * Preloading application

[9] * Listening on ssl://0.0.0.0:3000?key=/home/apprunner/keys/server.key&cert=/home/apprunner/keys/server.crt

[9] Use Ctrl-C to stop

[9] - Worker 0 (pid: 57) booted, phase: 0

[9] - Worker 1 (pid: 61) booted, phase: 0

However, if I simply change ENTRYPOINT to CMD (something I tried because I wish to deploy this to Heroku and Heroku demands that a CMD be present in the dockerfile) and run it the exact same way, I get this output:

/sbin/my_init: line 4: import: command not found

/sbin/my_init: line 5: import: command not found

/sbin/my_init: line 6: import: command not found

/sbin/my_init: line 7: import: command not found

/sbin/my_init: line 8: import: command not found

/sbin/my_init: line 9: import: command not found

/sbin/my_init: line 10: import: command not found

/sbin/my_init: line 11: import: command not found

/sbin/my_init: line 12: import: command not found

/sbin/my_init: line 13: import: command not found

/sbin/my_init: line 15: syntax error near unexpected token `('

/sbin/my_init: line 15: `KILL_PROCESS_TIMEOUT = int(os.environ.get('KILL_PROCESS_TIMEOUT', 5))'

Why is this different?  I thought these two commands, if specified individually, are equivalent.  I could not find anything on the web that indicates they run in different runtime environments. So I am quite confused.

Any help explaining this behavior would be much appreciated.