update docker file to use yarn and copy in yarn.lock (#52)

Dockerfile

@@ -6,15 +6,19 @@ WORKDIR /app
 
 # Copy package.json and package-lock.json to the working directory
 COPY package*.json ./
+COPY yarn.lock ./
 
 # Install any needed packages specified in package.json
-RUN npm install
+RUN npm install -g yarn
+
+# Install any needed packages specified in package.json
+RUN yarn
 
 # Copy the rest of your app's source code to /app
 COPY . .
 
 # Build the app
-RUN npm run build
+RUN yarn build
 
 # Define the command to run your app using CMD which defines your runtime
-CMD ["npm", "run", "start"]
+CMD ["yarn", "start"]