add .dockerignore file to reduce intermediate docker image size and build time

.dockerignore

@@ -0,0 +1,34 @@
+.dockerignore
+.git*
+Dockerfile
+
+# From .gitignore
+target
+.idea/
+test_site/public
+test_site_i18n/public
+docs/public
+
+small-blog
+medium-blog
+big-blog
+huge-blog
+extra-huge-blog
+small-kb
+medium-kb
+huge-kb
+
+current.bench
+now.bench
+*.zst
+
+# snapcraft artifacts
+snap/.snapcraft
+parts
+prime
+stage
+
+# nixos dependencies snippet
+shell.nix
+# vim temporary files
+**/.*.sw*