Merge pull request #20630 from tkopczynski/10084-cmd-entrypoint

Docs: add note about CMD and ENTRYPOINT commands

docs/reference/builder.md

@@ -950,6 +950,29 @@ If you then run `docker stop test`, the container will not exit cleanly - the
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+### Understand how CMD and ENTRYPOINT interact
+
+Both `CMD` and `ENTRYPOINT` instructions define what command gets executed when running a container.
+There are few rules that describe their co-operation.
+
+1. Dockerfile should specify at least one of `CMD` or `ENTRYPOINT` commands.
+
+2. `ENTRYPOINT` should be defined when using the container as an executable.
+
+3. `CMD` should be used as a way of defining default arguments for an `ENTRYPOINT` command
+or for executing an ad-hoc command in a container.
+
+4. `CMD` will be overridden when running the container with alternative arguments.
+
+The table below shows what command is executed for different `ENTRYPOINT` / `CMD` combinations:
+
+|                                | No ENTRYPOINT              | ENTRYPOINT exec_entry p1_entry                            | ENTRYPOINT ["exec_entry", "p1_entry"]          |
+|--------------------------------|----------------------------|-----------------------------------------------------------|------------------------------------------------|
+| **No CMD**                     | *error, not allowed*       | /bin/sh -c exec_entry p1_entry                            | exec_entry p1_entry                            |
+| **CMD ["exec_cmd", "p1_cmd"]** | exec_cmd p1_cmd            | /bin/sh -c exec_entry p1_entry exec_cmd p1_cmd            | exec_entry p1_entry exec_cmd p1_cmd            |
+| **CMD ["p1_cmd", "p2_cmd"]**   | p1_cmd p2_cmd              | /bin/sh -c exec_entry p1_entry p1_cmd p2_cmd              | exec_entry p1_entry p1_cmd p2_cmd              |
+| **CMD exec_cmd p1_cmd**        | /bin/sh -c exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd | exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd |
+
 ## VOLUME
 
     VOLUME ["/data"]