LiveView code reloading doesn't work when using Docker for local development

Your given code does not even run for me. I get permission denied error when container tries to run run.sh.

Your general approach seems a bit wrong. There should be a COPY statement somewhere in your Dockerfile. Also you should mix deps.get and mix deps.compile as part of the build.

Try this setup:

Dockerfile:

FROM elixir:1.10

RUN curl -sL https://deb.nodesource.com/setup_12.x | bash -
RUN apt update
RUN apt install -y git nodejs inotify-tools
RUN apt install -y chromium-driver

RUN mkdir -p /app
WORKDIR /app

RUN mix local.hex --force && \
  mix local.rebar --force && \
  mix archive.install --force hex phx_new

COPY mix.exs .
COPY mix.lock .

# copy the deps in dev environment for faster builds
COPY deps ./deps
RUN ["mix", "deps.get"]
RUN ["mix", "deps.compile"]

COPY assets ./assets
WORKDIR /app/assets
RUN ["npm", "install"]

WORKDIR /app

COPY config ./config
COPY lib ./lib
COPY seeds ./seeds
COPY priv ./priv
COPY test ./test
COPY dev/support ./dev/support

RUN ["mix", "compile"]

# compile deps in test environment for faster test runs when built
RUN export MIX_ENV=test && mix deps.compile

COPY ./run.sh ./run.sh
CMD ["/bin/bash", "entrypoint.sh"]

with a run.sh like this:

iex --sname app -S mix phx.server;

and a docker-compose.yml like this:

version: "3.8"
services:
  app:
    image: app:local
    build:
      context: .
      dockerfile: ./Dockerfile
    ports:
      - "4000:4000"
    volumes:
      - /app/assets/node_modules
      - ./assets:/app/assets:ro
      - ./config:/app/config:ro
      - ./lib:/app/lib:ro
      - ./priv:/app/priv
      - ./test:/app/test:ro
      - ./seeds:/app/seeds:ro
      - ./mix.exs:/app/mix.exs:ro
      - ./run.sh:/app/run.sh:ro
    stdin_open: true
    tty: true

I don’t know why your setup had the problem with config or mix.exs being “altered” but I suspect it has something to do with how your never actually COPY the files during your build.

Disclaimer: there is no one correct way to use docker. It depends on your own needs. What I have provided is good for local :dev environment, local debugging.

For development never use the COPY or ADD command in your Dockerfile, just map the the required folders in docker-compose.yml file. Also using using version 3.* in docker compose is only necessary if your are running the application in a docker swarm, otherwise just stick with the normal 2.* version that are more suitable for normal docker flow in development.

Why should you not COPY the files when building in dev?

Because you then need to rebuild the container each time you change a file, while mapping them in docker-compose.yml always reflects immediately in the docker container the changes done in host.

But if you mount them as volumes, like I did above, then the changes are reflected. You don’t have to rebuild the image.

So why are you then COPY them inside the container?

So I don’t have to build every time I run the container. With my approach, your built image has all the deps compiled and the code from the time you built the image. The dev can add new deps and edit the code and still use the same image because the volume mounting will change what is necessary. Note how I use ro with the volume mounting to get read-only access, so container cannot edit source code.

I don’t have to build every time I run the docker container, because I map the entire app I am working on to the container, thus I have the exact same dev flow I would have if I was not using docker.

I think that at this point you need to run the docker build command again.

Sure, if the deps are new, then you would have to rebuild. But what I look for is the smallest possible "boot"time. I don’t want anything being compiled or fetched just because I docker-compose down to go to the shop for 10 minutes. How often do you actually add/remove new deps when developing? Not even once per day, on average? If you don’t COPY anything during build phase then every time you spin up a new container, you have to fetch and compile all deps and compile the app? That seems like inefficient dev workflow to me. I would rather be able to spin up a new container in 5 seconds. I don’t mind rebuilding for new deps. That is a fair price for me.

Like I said, I think there are different approaches for using docker in dev environment.I don’t think there is one true way. I also don’t think it’s good to have absolute rules such as “never use the COPY…command…[for development]”.