travis: Support local sccache if not on bots

This commit configures local sccache directory to get used if you're running
builds locally to enjoy the benefits of caching when running inside the
containers.

src/ci/docker/run.sh

@@ -28,15 +28,23 @@ docker \
 mkdir -p $HOME/.cargo
 mkdir -p $root_dir/obj
 
+args=
+if [ "$SCCACHE_BUCKET" != "" ]; then
+    args="$args --env SCCACHE_BUCKET=$SCCACHE_BUCKET"
+    args="$args --env AWS_ACCESS_KEY_ID=$AWS_ACCESS_KEY_ID"
+    args="$args --env AWS_SECRET_ACCESS_KEY=$AWS_SECRET_ACCESS_KEY"
+else
+    mkdir -p $HOME/.cache/sccache
+    args="$args --env SCCACHE_DIR=/sccache --volume $HOME/.cache/sccache:/sccache"
+fi
+
 exec docker \
   run \
   --volume "$root_dir:/checkout:ro" \
   --volume "$root_dir/obj:/checkout/obj" \
   --workdir /checkout/obj \
   --env SRC=/checkout \
-  --env SCCACHE_BUCKET=$SCCACHE_BUCKET \
-  --env AWS_ACCESS_KEY_ID=$AWS_ACCESS_KEY_ID \
-  --env AWS_SECRET_ACCESS_KEY=$AWS_SECRET_ACCESS_KEY \
+  $args \
   --env CARGO_HOME=/cargo \
   --env LOCAL_USER_ID=`id -u` \
   --volume "$HOME/.cargo:/cargo" \