I’m trying to work with Docker to get my production environment to match my local development environment exactly. I’m following the Fly.io instructions here: Build, Deploy and Run an Elixir Application

In addition to bolting down the versions of Erlang and Elixir that I’m using, I would also like to bolt down the versions of NodeJS and NPM.

The Dockerfile suggested at Build, Deploy and Run an Elixir Application begins as follows:

###
### Fist Stage - Building the Release
###
FROM hexpm/elixir:1.12.1-erlang-24.0.1-alpine-3.13.3 AS build

# install build dependencies
RUN apk add --no-cache build-base npm

# prepare build dir
WORKDIR /app

But I’m concerned that the command RUN apk add --no-cache build-base npm won’t always result in the exact same version of NPM (and nodejs) running in the Docker image.

I’m confused because I thought that the whole point of Docker was to carefully control the software environment, so updating to the whatever the latest version on NPM is seems like it will constantly result in a different environment.

But this seems like a common command in Dockerfiles, so I think I must not understand something. What am I missing?