added a Dockerfile for the app

* app's frontend and backend are combined and served together from Node.js

app/.dockerignore

+client/node_modules
+server/node_modules
+.dockerignore
+Dockerfile

app/Dockerfile

+ARG NODEJS_VERSION='16.17.0'
+
+# ------------------------------------------------------------------
+# Base container to be used in the next stages
+
+FROM node:$NODEJS_VERSION-alpine AS base
+
+ENV \
+    PORT=3002 \
+    # For local dev - mongodb://host.docker.internal:27017/todo-app
+    MONGODB_URL=<should-be-dynamically-set> \
+    JWT_SECRET=<should-be-dynamically-set>
+
+# ------------------------------------------------------------------
+# Test stage
+FROM base AS test
+
+# client tests
+WORKDIR /client
+
+COPY ./client .
+
+RUN npm ci --no-audit --no-fund
+
+RUN npm run test
+
+# server tests
+WORKDIR /server
+
+COPY ./server/src ./src/
+
+COPY ./server/package*.json .
+
+RUN npm ci --no-audit --no-fund
+
+# MONGODB_URL and JWT_SECRET are available as env variables
+RUN npm run test
+
+# ------------------------------------------------------------------
+# Build stage
+FROM base AS build
+
+ARG APP_BUILD_PATH=/build
+
+# server build
+WORKDIR $APP_BUILD_PATH
+
+COPY ./server/src .
+
+COPY ./server/package*.json .
+
+RUN \
+    npm ci --prod --no-audit --no-fund \
+    && rm -rf ./package*
+
+# client build
+WORKDIR /client
+
+COPY ./client .
+
+RUN \
+    npm i --no-audit --no-fund \
+    && \
+    BUILD_PATH=..$APP_BUILD_PATH/public \
+    npm run build
+
+WORKDIR $APP_BUILD_PATH
+
+# MONGODB_URL and JWT_SECRET should be available as env variables
+CMD ["node", "./index.js"]
+
+EXPOSE $PORT