fix: Fix PostCSS config and Dockerfile

1. Rename postcss.config.js to postcss.config.cjs
   - Fixes CommonJS syntax in ES module project
   - Allows build to complete successfully

2. Remove --production flag from Dockerfile
   - Install ALL dependencies including sharp
   - Sharp required for Astro image optimization
   - Fixes sharp missing error

Both fixes enable successful Docker build and favicon to work.

astro.config.mjs

@@ -0,0 +1,11 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+
+import tailwindcss from '@tailwindcss/vite';
+
+// https://astro.build/config
+export default defineConfig({
+  vite: {
+    plugins: [tailwindcss()]
+  }
+});