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>
diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index 5d3bd4e..9d10de5 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -315,4 +315,4 @@
 
 # Add mkbootimg tool
 RUN git clone https://android.googlesource.com/platform/system/tools/mkbootimg /home/uboot/mkbootimg
-ENV PYTHONPATH "${PYTHONPATH}:/home/uboot/mkbootimg"
+ENV PYTHONPATH="${PYTHONPATH}:/home/uboot/mkbootimg"