fix(agent): derive @version from BUILD_ID so Burrito re-extracts per build

Burrito keys its on-host install directory on `{release}_erts-{ertsver}_{appver}`
and skips extraction when `_metadata.json` is already present for that version.
With a static `@version "0.1.0"` in mix.exs, every new build landed in the same
cached dir on the target host — silently running stale code.

Now @version resolves to `0.1.0+<BUILD_ID>` where BUILD_ID is the git short SHA
(or `dev-<timestamp>` fallback). scripts/build-linux.sh computes it on the host
and passes it through Dockerfile.build's ARG/ENV, so every commit produces a
distinct Burrito install dir and fresh extraction is guaranteed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agent/Dockerfile.build

@@ -28,6 +28,11 @@ COPY lib lib
 COPY config config
 
 RUN mix deps.compile
+
+# BUILD_ID is injected by scripts/build-linux.sh (typically the git short SHA).
+# Declared here — not earlier — so deps.compile stays cacheable across builds.
+ARG BUILD_ID=dev
+ENV BUILD_ID=${BUILD_ID}
 RUN mix release
 
 # Default: print the produced artifacts

agent/mix.exs

@@ -1,12 +1,12 @@
 defmodule ProxmoxAgent.MixProject do
   use Mix.Project
 
-  @version "0.1.0"
+  @version_base "0.1.0"
 
   def project do
     [
       app: :agent,
-      version: @version,
+      version: version(),
       elixir: "~> 1.17",
       start_permanent: Mix.env() == :prod,
       deps: deps(),
@@ -22,6 +22,15 @@ defmodule ProxmoxAgent.MixProject do
     ]
   end
 
+  # Version is built per-invocation so every commit produces a distinct Burrito
+  # install directory on the target host (`<release>_erts-<ertsver>_<version>`).
+  # Without this, Burrito sees an existing `_metadata.json` for the same version
+  # and skips extraction, silently running stale cached code.
+  defp version do
+    build_id = System.get_env("BUILD_ID") || "dev"
+    "#{@version_base}+#{build_id}"
+  end
+
   defp deps do
     [
       {:slipstream, "~> 1.1"},

agent/scripts/build-linux.sh

@@ -9,7 +9,14 @@ mkdir -p "$OUT"
 
 IMG="proxmox-monitor-agent-build:latest"
 
-docker build -f Dockerfile.build -t "$IMG" .
+# Unique per-build identifier. Git short SHA when available; otherwise a
+# timestamp fallback. Passed to Docker so @version in mix.exs expands to
+# `0.1.0+<BUILD_ID>` — this keys Burrito's on-host install dir so new builds
+# always re-extract instead of running stale cached code.
+BUILD_ID="$(git rev-parse --short=10 HEAD 2>/dev/null || echo "dev-$(date +%s)")"
+echo "Build ID: $BUILD_ID"
+
+docker build -f Dockerfile.build --build-arg BUILD_ID="$BUILD_ID" -t "$IMG" .
 docker run --rm -v "$OUT":/out "$IMG" sh -c 'cp -v burrito_out/* /out/'
 
 echo