Versioning of a nodejs project and dockerizing it with minimum image diff - docker

Question

The npm version is located at package.json.
I have a Dockerfile, simplified as follows:
FROM NODE:carbon
COPY ./package.json ${DIR}/
RUN npm install
COPY . ${DIR}
RUN npm build
Correct my understanding,
If ./package.json changes, is it true that the writable docker image layers changes are from 2 to 5?
Assuming that I do not have any changes on npm package dependencies,
How could I change the project version but I do not want docker rebuild image layer for RUN npm install ?

Answer 1

To sum up, the behavior you describe using Docker is fairly standard (as soon as package.json has changed and has a different hash, COPY package.json ./ will be trigerred again as well as each subsequent Dockerfile command).
Thus, the docker setup outlined in the official doc of Node.js does not alleviate this, and proposes the following Dockerfile:
FROM node:carbon
# Create app directory
WORKDIR /usr/src/app
# Install app dependencies
# A wildcard is used to ensure both package.json AND package-lock.json are copied
# where available (npm#5+)
COPY package*.json ./
RUN npm install
# If you are building your code for production
# RUN npm install --only=production
# Bundle app source
COPY . .
EXPOSE 8080
CMD [ "npm", "start" ]
But if you really want to find ways to avoid rerunning npm install from scratch most of the time, you could just maintain two different files package.json and, say, package-deps.json, import (and rename) package-deps.json and run npm install, then use the proper package.json afterwards.
And if you want to have more checks to be sure that the dependencies of both files are not out-of-sync, it happens that the role of file package-lock.json may be of some help, if you use the new npm ci feature that comes with npm 5.8 (cf. the corresponding changelog) instead of using npm install.
In this case, as the latest version of npm available in Docker Hub is npm 5.6, you'll need to upgrade it beforehand.
All things put together, here is a possible Dockerfile for this use case:
FROM node:carbon
# Create app directory
WORKDIR /usr/src/app
# Upgrade npm to have "npm ci" available
RUN npm install -g npm#5.8.0
# Import conf files for dependencies
COPY package-lock.json package-deps.json ./
# Note that this REQUIRES to run the command "npm install --package-lock-only"
# before running "docker build …" and also REQUIRES a "package-deps.json" file
# that is in sync w.r.t. package.json's dependencies
# Install app dependencies
RUN mv package-deps.json package.json && npm ci
# Note that "npm ci" checks that package.json and package-lock.json are in sync
# COPY package.json ./ # subsumed by the following command
COPY . .
EXPOSE 8080
CMD [ "npm", "start" ]
Disclaimer: I did not try the solution above on a realistic example as I'm not a regular node user, but you may view this as a useful workaround for the dev phase…