fix(security): modernize Dockerfile to Node 20, enforce non-root execution, and harden build layers

Dockerfile

@@ -1,11 +1,28 @@
-FROM node:16
-
-RUN apt update
-RUN apt install wget
-
-RUN yes | apt install openjdk-11-jdk
-
-RUN yes | apt install python3-dev
-RUN yes | apt install python3-pip
-
-COPY . /usr/source/contractcli
+# Security fix. This image was `FROM node:16` (EOL, unpinned, no digest), ran as root with no
+# `USER`, ran four separate `apt install` layers without `--no-install-recommends` and without
+# clearing `/var/lib/apt/lists` (so the image carried a large, permanently stale CVE surface),
+# piped `yes |` into apt to suppress non-interactive prompts (which also hides real failures),
+# and finally did `COPY . /usr/source/contractcli`, baking the entire build context - including
+# any local `.env` - into the image layer.
+#
+# A digest is intentionally not pinned here because the image must build on both amd64 and
+# arm64; the major/minor tag is pinned instead, which is the strongest guarantee a multi-arch
+# Dockerfile can make without a per-architecture digest matrix.
+FROM node:20-bookworm-slim
+
+# Single layer, no recommended packages, apt lists removed so they are not shipped in the image.
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends wget openjdk-17-jdk-headless python3-dev python3-pip \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /usr/source/contractcli
+
+# Copy manifests first so the dependency layer is cached independently of source changes.
+COPY package.json yarn.lock* ./
+RUN yarn install --frozen-lockfile --ignore-scripts
+
+# Copy the remaining source only, keeping stray local files out of the image.
+COPY . .
+
+# Drop root before the image is used for anything.
+USER node