feat: create bash-completion file

This only completes the command names - not any arguments - and also relies on a completion cache as the response from Dokku for help output can be slow.

Closes #2095

contrib/bash-completion

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+
+_dokku_complete_goals_fetch() {
+  dokku --quiet help --all | awk '/^    /{ print $1 }' | sort > "/var/cache/dokku-completion"
+}
+
+_dokku_complete_verify() {
+  declare desc="shows bash completion"
+  declare COMMAND="$1"
+  local COMMANDS=""
+
+  if [[ ! -f "/var/cache/dokku-completion" ]] || [[ -s "/var/cache/dokku-completion" ]]; then
+    _dokku_complete_goals_fetch
+  fi    
+
+  _dokku_complete_goals=$(cat "/var/cache/dokku-completion")
+
+  while read -r fn; do
+    if [[ -z "$COMMAND" ]] || [[ "$fn" == $COMMAND* ]]; then
+      COMMANDS="${COMMANDS};${fn}"
+    fi
+  done <<< "$_dokku_complete_goals"
+
+  echo "${COMMANDS#";"}"
+}
+
+have dokku &&
+_dokku_complete() {
+  local cur prev
+  _get_comp_words_by_ref -n : cur
+  opts=$(_dokku_complete_verify "${cur}")
+  COMPREPLY=($(IFS=";" compgen -W "$opts" -- "$cur"))
+  __ltrim_colon_completions "$cur"
+} &&
+complete -F _dokku_complete dokku
+
+if [[ ! -f "/var/cache/dokku-completion" ]] || [[ -s "/var/cache/dokku-completion" ]]; then
+  have dokku && export -f _dokku_complete_goals_fetch && nohup bash -c _dokku_complete_goals_fetch >/dev/null 2>&1 &
+fi