Update README

README.md

@@ -4,14 +4,29 @@ Building modern Zig from the original C++ implementation
 This repository explores building Zig from the C++ implementation without using
 binary blobs. [Forum post goes with this][1].
 
+Clone zig and prune binary files from history:
+
+```
+$ git clone --reference zig --dissociate https://github.com/ziglang/zig zig-nobinaries
+$ cd zig-nobinaries
+$ git filter-repo --prune-empty=never --prune-degenerate never --invert-paths --path-glob 'stage1/zig1.wasm*' --path stage1/zig1.c
+```
+
+Now build an isolated container with a bunch of zig historic dependencies:
+
 ```
 $ docker build -t zig-repro .
 $ docker run -ti --rm \
     -v ~/code:/x \
     -v /home/$USER/.cache/zig:/home/$USER/.cache/zig \
-    -w /x \
+    -w /x/zig \
     zig-repro \
-    sh -c "useradd -s /bin/bash $USER && cd zig && su $USER; bash"
+    sh -c "useradd -s /bin/bash -u $UID $USER && su $USER; bash"
+```
+
+And run the thing in the container:
+
+```
 $ ../zig-repro/run
 ```