FIXME: Menu with running containers needs work and exec command broken

[gitayam]

What’s happening?
The menu that lists running containers isn’t behaving correctly, and the dexec() function never actually runs the command inside the container.

Current code:

dexec() {
  # … your implementation …
}

Expected behavior:
Selecting a container and a command should invoke that command inside the container’s shell.

Actual behavior:
Nothing happens (or an error is thrown).

Steps to reproduce:

1. Run your menu script
2. Pick a running container
3. Choose a command to execute
4. Observe that nothing runs

Environment:

• OS:
• Shell: <bash/zsh/etc>
• Script version: <git commit/tag>

[gitayam]

Part of 28a1a27