perf(docker): reuse dependency layers and preserve build caches

src/lib/docker-build-contract.test.ts

@@ -0,0 +1,28 @@
+import { readFileSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+
+const dockerfile = readFileSync("Dockerfile", "utf8");
+
+describe("Docker build cache", () => {
+	it("installs frozen dependencies before copying application source", () => {
+		const manifests = dockerfile.indexOf(
+			"COPY package.json *lock* pnpm-workspace.yaml .npmrc ./",
+		);
+		const install = dockerfile.indexOf("pnpm install --frozen-lockfile");
+		const source = dockerfile.indexOf("COPY . .");
+		expect(manifests).toBeGreaterThan(-1);
+		expect(install).toBeGreaterThan(manifests);
+		expect(source).toBeGreaterThan(install);
+	});
+	it("retains the package store and Next compiler cache across source changes", () => {
+		expect(dockerfile).toContain("id=epicnext-pnpm,target=/pnpm/store");
+		expect(dockerfile).toContain("--store-dir=/pnpm/store");
+		expect(dockerfile).toContain("id=epicnext-next,target=/app/.next/cache");
+	});
+	it("ships standalone output without a redundant dependency pruning step", () => {
+		expect(dockerfile).toContain("/app/.next/standalone ./");
+		expect(dockerfile).not.toContain("pnpm prune --prod");
+		expect(dockerfile).not.toContain("npm prune --production");
+		expect(dockerfile).not.toContain("yarn install --production");
+	});
+});