README: use apt-get --assume-yes in Dockerfile

This little change makes it even easier for someone to create their own
Dockerfile: they simply copy an example and start listing the packages
they need. Without `--assume-yes`, `apt-get install` will try to
interactively ask for confirmation, and fail since `docker build` runs
it in a non-interactive environment.

README.md

@@ -116,7 +116,7 @@ FROM rustembedded/cross:aarch64-unknown-linux-gnu-0.1.16
 
 RUN dpkg --add-architecture arm64 && \
     apt-get update && \
-    apt-get install libfoo:arm64
+    apt-get install --assume-yes libfoo:arm64
 ```
 
 ```