Add emsdk_env when building docker image.

docker/Dockerfile

diff --git a/docker/Dockerfile b/docker/Dockerfile
index f58c5ce..354fd37 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -35,6 +35,7 @@
     && ./emsdk activate ${EMSCRIPTEN_VERSION} \
     && chmod 777 ${EMSDK}/upstream/emscripten \
     && chmod -R 777 ${EMSDK}/upstream/emscripten/cache \
+    && . ./emsdk_env.sh \
     && echo "int main() { return 0; }" > hello.c \
     && ${EMSDK}/upstream/emscripten/emcc -c hello.c \
     && cat ${EMSDK}/upstream/emscripten/cache/sanity.txt \