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>

contrib/completion/bash/docker

@@ -1277,7 +1277,7 @@ _docker_exec() {
 
 	case "$cur" in
 		-*)
-			COMPREPLY=( $( compgen -W "--detach -d --detach-keys --help --interactive -i --privileged -t --tty -u --user" -- "$cur" ) )
+			COMPREPLY=( $( compgen -W "--detach -d --detach-keys -e --env --help --interactive -i --privileged -t --tty -u --user" -- "$cur" ) )
 			;;
 		*)
 			__docker_complete_containers_running

contrib/completion/zsh/_docker

@@ -1679,6 +1679,7 @@ __docker_subcommand() {
                 $opts_help \
                 $opts_attach_exec_run_start \
                 "($help -d --detach)"{-d,--detach}"[Detached mode: leave the container running in the background]" \
+                "($help -e --env)"{-e,--env}"[Set environment variables]" \
                 "($help -i --interactive)"{-i,--interactive}"[Keep stdin open even if not attached]" \
                 "($help)--privileged[Give extended Linux capabilities to the command]" \
                 "($help -t --tty)"{-t,--tty}"[Allocate a pseudo-tty]" \

docs/reference/commandline/exec.md

@@ -14,6 +14,7 @@ Run a command in a running container
 Options:
   -d, --detach         Detached mode: run command in the background
       --detach-keys    Override the key sequence for detaching a container
+  -e, --env=[]         Set environment variables
       --help           Print usage
   -i, --interactive    Keep STDIN open even if not attached
       --privileged     Give extended privileges to the command

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