Bump docker/setup-buildx-action to v4 in examples and README

The docker-container:// endpoint requires v4; v3 fails with "context deadline exceeded" during bootstrap.

Author: dash14

.github/workflows/example-audit.yml

@@ -16,7 +16,7 @@ jobs:
           proxy_mode: audit
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
         with:
           driver: remote
           endpoint: docker-container://buildcage

.github/workflows/example-restrict.yml

@@ -19,7 +19,7 @@ jobs:
             fonts.googleapis.com:443
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
         with:
           driver: remote
           endpoint: docker-container://buildcage

README.md

@@ -96,7 +96,7 @@ jobs:
           proxy_mode: audit  # Log everything, block nothing
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
         with:
           driver: remote
           endpoint: docker-container://buildcage
@@ -147,7 +147,7 @@ jobs:
             fonts.googleapis.com:443
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
         with:
           driver: remote
           endpoint: docker-container://buildcage
@@ -216,7 +216,7 @@ Pass the container name to [`docker/setup-buildx-action`](https://github.com/doc
 
 ```yaml
 - name: Set up Docker Buildx
-  uses: docker/setup-buildx-action@v3
+  uses: docker/setup-buildx-action@v4
   with:
     driver: remote
     endpoint: docker-container://buildcage