1

When building an image of a docker container I need to access a service on host localhost.

It looks like --add-host option for docker command updates /etc/hosts both at image build time and container runtime.

The analogous option for docker-compose command, extra_hosts seems to work only at container runtime. How do I enable it at docker image build time?

Looks like there feature request for that: https://github.com/docker/compose/issues/7323, but not implemented yet. What would be workaround until this is solved?

Minimal working example

Here's my Dockerfile:

FROM python:3.8-slim-buster
RUN grep "host.docker.internal" /etc/hosts || echo "Missing"
CMD grep "host.docker.internal" /etc/hosts || echo "Missing"

and docker-compose.yml:

version: "3.9"
services:
  test:
    build: .
    extra_hosts:
      - "host.docker.internal:host-gateway"

Running docker:

docker build . --add-host host.docker.internal:host-gateway  --no-cache -t test \
    && docker run --add-host host.docker.internal:host-gateway test

prints:

172.17.0.1      host.docker.internal
...
172.17.0.1      host.docker.internal
...

while:

docker-compose build --no-cache && docker-compose up

returns

Missing
...
test_1  | 172.17.0.1    host.docker.internal
...

Motivation

I have a Flask app that depends on my private Python package that I install during docker image build. I store my private Python packages in my local devpi instance hosted on the same machine I use for building docker image. Hence, my Flask app pyproject.toml points to the devpi instance:

url = "http://localhost:3141/testuser/dev/+simple"

or rather

url = "http://host.docker.internal:3141/testuser/dev/+simple"

1 Answer 1

0

The solution is to use Compose V2 and then:

docker compose build --no-cache && docker compose up

(note dashes replaced with spaces).

Here's specific line in implementation.

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