Add git-lfs package to emsdk Docker image (#665)

Having LFS support already in the image saves some build time and allows a more consistent experience to other CI build images.

docker/Dockerfile

diff --git a/docker/Dockerfile b/docker/Dockerfile
index f58c5ce..48ff1a8 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -101,6 +101,7 @@
         zip \
         unzip \
         git \
+        git-lfs \
         ssh-client \
         build-essential \
         make \