introduce —workdir option for docker exec

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

docs/reference/commandline/exec.md

@@ -29,6 +29,7 @@ Options:
       --privileged     Give extended privileges to the command
   -t, --tty            Allocate a pseudo-TTY
   -u, --user           Username or UID (format: <name|uid>[:<group|gid>])
+  -w, --workdir        Working directory inside the container  
 ```
 
 ## Description
@@ -86,6 +87,20 @@ This will create a new Bash session in the container `ubuntu_bash` with environm
 variable `$VAR` set to "1". Note that this environment variable will only be valid 
 on the current Bash session.
 
+By default `docker exec` command runs in the same working directory set when container was created.
+
+```bash
+$ docker exec -it ubuntu_bash pwd
+/
+```
+
+You can select working directory for the command to execute into
+
+```bash
+$ docker exec -it -w /root ubuntu_bash pwd
+/root
+```
+
 
 ### Try to run `docker exec` on a paused container