As you probably already know, Docker makes a pretty aggresive use of its cache and the criteria used to decide whether a layer can be reused is quite simple and well described in Dockerfile best practices.
Basically Docker just checks that the instruction in the Dockerfile has not changed and if that’s the case it will reuse the cached image. For ADD and COPY instructions it will check contents to see if something has changed.
In most Node projects we don’t install or remove a package often so it’s desirable to have our node_modules folders cached across builds. What we want is our Docker build to output something like this:
Step 9/17 : RUN cd /usr/src/app && npm ci
---> Using cache
---> 37c1416a0392
Note that I’m using npm ci, which is the command that guarantees that you’re going to get exactly what your package-lock.json says: https://docs.npmjs.com/cli/ci
The technique is quite simple and consists in copying package.json and package-lock.json and issuing npm ci before adding/copying your code:
Dockerfile
COPY package.json /usr/src/app
COPY package-lock.json /usr/src/app
RUN cd /usr/src/app && npm ci
That way, if both your package.json and package-lock.json didn’t change, the cache will be valid when Docker reaches npm ci and your build will be blazingly fast.
To give more context here’s a sample Dockerfile with it:
Dockerfile
COPY package.json /usr/src/app
FROM marianobe/node-base:latest
EXPOSE 3000 3001 3002
RUN npm install -g --unsafe-perm @angular/cli pushstate-server
RUN mkdir -p /usr/src/app/client
WORKDIR /usr/src/app
# Loopback app
COPY package.json /usr/src/app
COPY package-lock.json /usr/src/app
RUN cd /usr/src/app && npm ci
# Angular app
COPY ./client/package.json /usr/src/app/client
COPY ./client/package-lock.json /usr/src/app/client
RUN cd /usr/src/app/client && npm ci
COPY . /usr/src/app
WORKDIR /usr/src/app/client
RUN npm run build