This adds a new core command, `docker-exec`, which allows the user to
exec into an already-running container.
- Fixes#6566
- Fixes#5193
- Fixes#4904
- Fixes#4057
- Fixes#4179
- Fixes#4903
When VAGRANT_HOME is set to a different path than $HOME/.vagrant.d, bash
completions regarding box names fails with an error like:
"$ vagrant init <TAB> find: `/home/mert/.vagrant.d/boxes': No such file or directory"
This commit makes completion.sh respect VAGRANT_HOME if it is set. It will continue to use $HOME/.vagrant.d otherwise.