fix(rust): Check /proc for health checks (#3250)

Debian slim is slimmer than we could ever have imagined.

.github/README_CI.md

@@ -59,3 +59,15 @@ difference using a larger runner.
 We maintain a baremetal testbed for running our end-to-end test suite. See
 [the `e2e`](../e2e) directory. Please don't target those runners unless you're
 specifically trying to run workflows that require a baremetal runner.
+
+## Busting the GCP Docker layer cache
+
+If you find yourself hitting strange Docker image issues like Rust binaries
+failing to start inside Docker images, you may need to bust the GCP layer cache.
+
+To do so:
+
+- Login to [GCP](console.cloud.google.com)
+- Ensure `firezone-staging` project is selected
+- Navigate to the artifact registry service
+- Delete all image versions for the appropriate `cache/` image repository

.github/workflows/cd.yml

@@ -211,6 +211,7 @@ jobs:
       - name: Build and push release Docker images by digest
         id: build
         uses: docker/build-push-action@v5
+        target: release
         with:
           platforms: ${{ matrix.arch.platform }}
           build-args: |