Mount a host directory as container's /tmp (#13)

This way, when Clang crashes inside the container, auto-generated
reproducers are easily accessible on the host.

Author: atrosinenko

.gitignore

@@ -4,3 +4,4 @@ ccache*/
 .build*/
 /src/
 /inst/
+/tmp/

build.sh

@@ -104,6 +104,7 @@ build_in_docker() {
       --volume "$ROOT/output:$OUTPUT_DIR:rw" \
       --volume "$ROOT/ccache:$CCACHE_DIR:rw" \
       --volume "$ROOT/src:$SRC_DIR:ro" \
+      --volume "$ROOT/tmp:/tmp:rw" \
       --tmpfs "$DOCKER_BUILD_INSTALL_DIR:rw,exec,size=2G" \
       --tmpfs "$BUILD_TMP:rw,exec,size=5G" \
       "$DOCKER_IMAGE_NAME" "$REPO_ROOT/scripts/build-in-docker.sh"