Docker gotcha with building a Dockerfile in sub directory

tl;dr; Watch out for .dockerignore causing no such file or directory when building Docker images

First I tried to use docker-compose:

▶ docker-compose build ui
Building ui
Step 1/8 : FROM node:9
 ---> 29831ba76d93
Step 2/8 : ADD ui/package.json /package.json
ERROR: Service 'ui' failed to build: ADD failed: stat /var/lib/docker/tmp/docker-builder079614651/ui/package.json: no such file or directory

What the heck? Did I typo the name in the ui/Dockerfile?

The docker-compose.yml directive for this was:

yaml
  ui:
    build:
      context: .
      dockerfile: ui/Dockerfile
    environment:
      - NODE_ENV=development
    ports:
      - "3000:3000"
      - "35729:35729"
    volumes:
      - $PWD/ui:/app
    command: start

I don't know if it's the awesomest way to do docker-compose but I did copy this exactly from a different (working!) project. That other project called the web stuff frontend instead of ui in this project.

The Dockerfile looked like this:

FROM node:9

ADD ./ui/package.json ./
ADD ./ui/package-lock.json ./

RUN npm install

EXPOSE 3000
EXPOSE 35729

CMD [ "npm", "start" ]

Let's try without docker-compose. Or rather, do with docker what docker-compose does for me automatically.

▶ docker build ui -f ui/Dockerfile
Sending build context to Docker daemon  158.2MB
Step 1/8 : FROM node:9
 ---> 29831ba76d93
Step 2/8 : ADD ui/package.json /package.json
ADD failed: stat /var/lib/docker/tmp/docker-builder001494654/ui/package.json: no such file or directory

So I thought I perhaps have misunderstood how relative paths worked. I tried EVERYTHING!

I tried changing to docker build . -f ui/Dockerfile. No luck.

I tried all sorts of combinations of this with also changing the line to ADD ui/package.json ... or ADD /ui/package.json ... or ADD ./package.json ... or ADD package.json .... Nothing worked.

Finally I got it to work by doing

▶ cd ui
▶ docker build . -f Dockerfile

but for that to work I had to remove all references of the directory name ui in the Dockerfile. Nice, but this is not going to work in docker-compose.yml since that starts outside the directory ./ui/.

Sigh!

So then I learned about contexts in docker. Well, I skimmed the docs rapidly. To keep things clean it's a good idea to do things within the directory that matters. So I made this change in the docker-compose.yml:

  ui:
    build:
      context: ui
      dockerfile: Dockerfile
    environment:
      - NODE_ENV=development
    ports:
      - "3000:3000"
      - "35729:35729"
    volumes:
      - $PWD/ui:/app
    command: start

(Note the build.context: and build.dockerfile:)

Still doesn't work! 😫 Still various variations of no such file or directory.

The solution

Turns out, in projectroot/.dockerignore it had ui/ as a line entry!!!

I believe this project used to do some of the Python stuff with Docker and the React web app was done "locally" on the host. And since the ui/node_modules directory is so huge someone decided it was smart of avoid Docker mounting that.

Now the .dockerignore has .ui/node_modules and now everything works. I can build it with plain docker and docker-compose from outside the directory.

Perhaps I should have spent the time I spent writing this blog post to instead file a structured GitHub issue on Docker itself somewhere. I.e. that it should have warned be better. Any takers?