Toolchain: Add ccache to Dockerfile

Following up on 2d38d56e, we were missing this in our Dockerfile.

Toolchain/Dockerfile

@@ -5,6 +5,7 @@ ENV DEBIAN_FRONTEND=noninteractive
 RUN apt-get update -y \
     && apt-get install -y \
         build-essential \
+        ccache \
         cmake \
         curl \
         genext2fs \