implement exec

Resolves #593

Signed-off-by: Tomas Tomecek <ttomecek@redhat.com>

docs/reference/exec.md

@@ -0,0 +1,29 @@
+<!--[metadata]>
++++
+title = "exec"
+description = "exec"
+keywords = ["fig, composition, compose, docker, orchestration, cli,  exec"]
+[menu.main]
+identifier="exec.compose"
+parent = "smn_compose_cli"
++++
+<![end-metadata]-->
+
+# exec
+
+```
+Usage: exec [options] SERVICE COMMAND [ARGS...]
+
+Options:
+-d                Detached mode: Run command in the background.
+--privileged      Give extended privileges to the process.
+--user USER       Run the command as this user.
+-T                Disable pseudo-tty allocation. By default `docker-compose exec`
+                  allocates a TTY.
+--index=index     index of the container if there are multiple
+                  instances of a service [default: 1]
+```
+
+This is equivalent of `docker exec`. With this subcommand you can run arbitrary
+commands in your services. Commands are by default allocating a TTY, so you can
+do e.g. `docker-compose exec web sh` to get an interactive prompt.