devtools/docker: upgrade to 3.4 compose file format
Upgrading to version 3.4 allows us to use extension fields to factor out
environment variables.
This is done in the next CL.
Trust: Julie Qiu
Run-TryBot: Julie Qiu
TryBot-Result: kokoro
# Copyright 2021 The Go Authors. All rights reserved.
# Use of this source code is governed by a BSD-style
# license that can be found in the LICENSE file.
-version: '3'
+version: '3.4'
services:
allbash:
# Allocate a pseudo-tty, which sets the TERM
