feat(agent): docker-based cross-compile for linux binaries

agent/Dockerfile.build

@@ -0,0 +1,32 @@
+# Reproducible Burrito build environment for the agent.
+# Produces linux_amd64 + linux_arm64 binaries into /work/agent/burrito_out.
+# Keep elixir/OTP in sync with what the project compiles against locally
+# (see `elixir --version` — currently Elixir 1.19 on OTP 28).
+FROM elixir:1.19-otp-28 AS build
+
+ENV MIX_ENV=prod DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+      build-essential git ca-certificates curl xz-utils unzip 7zip \
+    && rm -rf /var/lib/apt/lists/*
+
+# Zig (Burrito needs it for cross-compile)
+ARG ZIG_VERSION=0.13.0
+RUN curl -fsSL https://ziglang.org/download/${ZIG_VERSION}/zig-linux-$(uname -m)-${ZIG_VERSION}.tar.xz \
+    | tar -xJ -C /opt && ln -s /opt/zig-linux-*/zig /usr/local/bin/zig
+
+WORKDIR /work/agent
+RUN mix local.hex --force && mix local.rebar --force
+
+# Copy sources last for layer caching
+COPY mix.exs mix.lock ./
+RUN mix deps.get --only prod
+
+COPY lib lib
+COPY config config
+
+RUN mix deps.compile
+RUN mix release
+
+# Default: print the produced artifacts
+CMD ["sh", "-c", "ls -la burrito_out/"]