Add Dockerfile-based Dokku deployment for monorepo layout

Uses a multi-stage Docker build that copies both app/ and blogex/,
preserving the path dependency. Includes release scripts, migration
module, and a sample Dokku setup script.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,31 @@
+# Git
+.git
+
+# Build artifacts
+app/_build
+app/deps
+blogex/_build
+blogex/deps
+
+# Dev/test only
+app/test
+blogex/test
+app/.formatter.exs
+blogex/.formatter.exs
+
+# IDE
+.devcontainer
+.claude
+
+# Documentation
+*.md
+!app/README.md
+
+# Misc
+app/tmp
+app/cover
+app/doc
+blogex/doc
+
+# Dokku setup (may contain secrets)
+dokku-setup.sh