docker: Install pyelftools for builds

Binman needs this module to build sandbox_vpl and it is needed elsewhere
in CI.

Add it to the docker file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xpyron.glpk@gmx.de>
diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index 84b7777..42ab812 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -97,6 +97,7 @@
 	python3 \
 	python3-dev \
 	python3-pip \
+	python3-pyelftools \
 	python3-sphinx \
 	python3-virtualenv \
 	rpm2cpio \