feat: prime the bash-completion output

While not ideal, users can manually update the completion-cache by reconfiguring the installed package, or updating Dokku.

debian/postinst

@@ -80,6 +80,9 @@ case "$1" in
     echo "Install all core plugins"
     dokku plugin:install --core
 
+    echo "Creating bash-completion cache"
+    dokku --quiet help --all | awk '/^    /{ print $1 }' | sort > "/var/cache/dokku-completion"
+
     rm -f ${DOKKU_ROOT}/VERSION
     cp ${DOKKU_LIB_ROOT}/STABLE_VERSION ${DOKKU_ROOT}/VERSION
 

rpm/dokku.postinst

@@ -37,6 +37,9 @@ setup-plugins() {
 
   echo "Install all core plugins"
   dokku plugin:install --core
+
+  echo "Creating bash-completion cache"
+  dokku --quiet help --all | awk '/^    /{ print $1 }' | sort > "/var/cache/dokku-completion"
 }
 
 setup-sshcommand() {