Phoenix + Tailwind + Docker issues


Update 10-15-2023

This is no longer an issue since Phoenix started bundling Tailwind.

The problem

I was having issues with tailwind building correctly only in production and only when built in a Docker container.

I followed an article I found online to get postcss and tailwind configured correctly so that it only builds the classes required by the application (we don’t want a huge css file).

Using Tailwind with Phoenix

My tailwind.config.js looked like this:

module.exports = {    purge: [
  enabled: process.env.NODE_ENV === 'production',
  mode: 'jit',
  darkMode: false, // or 'media' or 'class'
  theme: {
    extend: {
      container: (theme) => ({
        center: true,
        padding: theme("spacing.4"),
        screens: {
          sm: "100%",
          md: "100%",
          lg: "1024px",
          xl: "1280px"
  variants: {
    extend: {},
  plugins: [

Here we are telling tailwind to search through our .ex, .leex, .eex and .js files for relevent classes and purge anything not used.

This all worked perfect in development and local testing.

I add a Dockerfile to use as my production deployment stategy. I started with the version of Phoenix documented Dockerfile and tweaked it a little to use the latest version of Elixir. Here is what I ended up with that was not compiling tailwind correctly:

Phoenix documented Dockerfile

FROM elixir:1.12.1-alpine AS build  
RUN apk add --no-cache build-base npm git python3


RUN mix local.hex --force && \
    mix local.rebar --force


COPY mix.exs mix.lock ./
COPY config config
RUN mix do deps.get, deps.compile

# build assets
COPY assets/package.json assets/package-lock.json ./assets/
RUN npm --prefix ./assets ci --progress=false --no-audit --loglevel=error

COPY priv priv
COPY assets assets

RUN npm run --prefix ./assets deploy
RUN mix phx.digest

COPY lib lib
# uncomment COPY if rel/ exists
# COPY rel rel
RUN mix do compile, release

# prepare release image
FROM alpine:3.13 AS app
RUN apk add --no-cache openssl ncurses-libs libstdc++


RUN chown nobody:nobody /app

USER nobody:nobody

COPY --from=build --chown=nobody:nobody /app/_build/prod/rel/my_app ./


CMD trap 'exit' INT; ./bin/my_app eval "MyApp.Release.migrate()" && ./bin/my_app start

The solution

I tried many different things to fix the issue. I tried installing a specific version of node in the image - building a development build with webpack - but what it came down to was an order of operations bug.

Remember the tailwind config from above? It says to purge any tailwind classes that haven’t been used in our template files. Well, our image doesn’t have any template files at the time of running npm run --prefix ./assets deploy because we haven’t copied our lib folder over to the image yet!

So the simple fix was to move COPY lib lib up in the Dockerfile before running npm deploy.