docker-exec: add page (#4518)

pages/common/docker-exec.md

@@ -0,0 +1,24 @@
+# docker exec
+
+> Execute a command on an already running Docker container.
+> More information: <https://docs.docker.com/engine/reference/commandline/exec/>.
+
+- Enter an interactive shell session on an already-running container:
+
+`docker exec -it {{container_name}} {{/bin/bash}}`
+
+- Run a command in the background (detached) on a running container:
+
+`docker exec -d {{container_name}} {{command}}`
+
+- Select the working directory for a given command to execute into:
+
+`docker exec -it -w {{path/to/directory}} {{container_name}} {{command}}`
+
+- Run a command in background on existing container but keep `stdin` open:
+
+`docker exec -i -d {{container_name}} {{command}}`
+
+- Set an environment variable in a running bash session:
+
+`docker exec -it -e {{variable_name}}={{value}} {{container_name}} {{/bin/bash}}`