add vendor dir to dockerfile

Dockerfile

@@ -41,6 +41,7 @@ ENV MIX_ENV="prod"
 
 # install mix dependencies
 COPY mix.exs mix.lock ./
+COPY vendor vendor
 RUN --mount=type=cache,target=/root/.hex \
     --mount=type=cache,target=/root/.mix \
     mix deps.get --only $MIX_ENV

vendor/README.md

@@ -9,6 +9,17 @@ These dependencies are vendored to:
 - Ensure consistent builds without external dependencies
 - Enable offline development
 
+## Docker Build
+
+**IMPORTANT**: The `vendor/` directory must be copied into the Docker build before running `mix deps.get`.
+
+In `Dockerfile`, ensure this order:
+```dockerfile
+COPY mix.exs mix.lock ./
+COPY vendor vendor          # ← Must be before mix deps.get
+RUN mix deps.get --only $MIX_ENV
+```
+
 ## Vendored Packages
 
 ### Oban Pro