Fix Alloy binary on Alpine: add libc6-compat

The grafana/alloy image is Ubuntu-based (glibc), but our container
uses nginx:alpine (musl). The binary exists but fails with "not found"
because the glibc dynamic linker is missing. libc6-compat provides
the compatibility shim.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

fly/Dockerfile

@@ -7,9 +7,10 @@ COPY --from=docker.io/tailscale/tailscale:stable \
     /usr/local/bin/tailscale /usr/local/bin/tailscale
 
 RUN mkdir -p /var/run/tailscale /var/lib/tailscale \
-    && apk add --no-cache iptables ip6tables
+    && apk add --no-cache iptables ip6tables \
+    && apk add --no-cache libc6-compat
 
-# Copy Alloy binary from official image
+# Copy Alloy binary from official image (Ubuntu-based, needs libc6-compat)
 COPY --from=docker.io/grafana/alloy:v1.5.1 \
     /bin/alloy /usr/local/bin/alloy