[docker] Fix unmatched quote problem in here-document on older versions of bash

Summary:
When outputing usage, emit here-document directly instead of
saving in a variable first -- avoids problem with bash 3.2.57 where an
unmatched ' in the here-document results in the following error:

./build_docker_image.sh: line 135: unexpected EOF while looking for matching `''

bash --version
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)

Differential Revision: https://reviews.llvm.org/D36064

llvm-svn: 309568