docker: Fix LegacyKeyValueFormat warning with PYTHONPATH

Fix a warning due to the syntax used for PYTHONPATH:

   LegacyKeyValueFormat: "ENV key=value" should be used instead of
      legacy "ENV key value" format (line 304)

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
1 file changed