Make it clear who is doing variable expansion
Add sentece to RUN, CMD, and ENTRYPOINT exec sections making it clear
that it is the shell doing the environment variable expansion.
@@ -541,6 +541,9 @@ RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'
> `RUN [ "echo", "$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: `RUN [ "sh", "-c", "echo $HOME" ]`.
+> When using the exec form and executing a shell directly, as in the case for
+> the shell form, it is the shell that is doing the environment variable
+> expansion, not docker.
>
> **Note**:
> In the *JSON* form, it is necessary to escape backslashes. This is
@@ -607,6 +610,9 @@ instruction as well.
> `CMD [ "echo", "$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 $HOME" ]`.
+> When using the exec form and executing a shell directly, as in the case for
+> the shell form, it is the shell that is doing the environment variable
+> expansion, not docker.
When used in the shell or exec formats, the `CMD` instruction sets the command
to be executed when running the image.
@@ -1075,6 +1081,9 @@ sys 0m 0.03s
> `ENTRYPOINT [ "echo", "$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: `ENTRYPOINT [ "sh", "-c", "echo $HOME" ]`.
+> When using the exec form and executing a shell directly, as in the case for
+> the shell form, it is the shell that is doing the environment variable
+> expansion, not docker.
### Shell form ENTRYPOINT example
