polish: specify node version in dockerfile

Dockerfile

@@ -1,9 +1,9 @@
-FROM node:alpine AS build
+FROM node:12-alpine AS build
 WORKDIR /src
 COPY . .
 RUN npm ci && npm run build
 
-FROM node:alpine
+FROM node:12-alpine
 WORKDIR /app
 COPY --from=build /src/dist .
 ENTRYPOINT node /app/index.js