Exec: Add ability to set environment variables

Keeping the current behavior for exec, i.e., inheriting
variables from main process. New variables will be added
to current ones. If there's already a variable with that
name it will be overwritten.

Example of usage: docker exec -it -e TERM=vt100 <container> top

Closes #24355.

Signed-off-by: Jonh Wendell <jonh.wendell@redhat.com>

man/docker-exec.1.md

@@ -8,6 +8,7 @@ docker-exec - Run a command in a running container
 **docker exec**
 [**-d**|**--detach**]
 [**--detach-keys**[=*[]*]]
+[**-e**|**--env**[=*[]*]]
 [**--help**]
 [**-i**|**--interactive**]
 [**--privileged**]
@@ -32,6 +33,12 @@ container is unpaused, and then run
 **--detach-keys**=""
   Override the key sequence for detaching a container. Format is a single character `[a-Z]` or `ctrl-<value>` where `<value>` is one of: `a-z`, `@`, `^`, `[`, `,` or `_`.
 
+**-e**, **--env**=[]
+   Set environment variables
+
+   This option allows you to specify arbitrary environment variables that are
+available for the command to be executed.
+
 **--help**
   Print usage statement