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.

postcss.config.cjs

@@ -0,0 +1,6 @@
+module.exports = {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};