Get Host IP in Dockerfile - docker

Question

I have a line in my Dockerfile:
&& echo "xdebug.remote_host=192.168.0.216" >> /usr/local/etc/php/conf.d/xdebug.ini`
I want to make the IP dynamic. How would I get the host IP in there?

Answer 1

You need to use build-time variables (–build-arg).
This flag allows you to pass the build-time variables that are accessed like regular environment variables in the RUN instruction of the Dockerfile.
So, Dockerfile is modified to:
ARG IP_ADDRESS
RUN ... && echo "xdebug.remote_host=$IP_ADDRESS" >> /usr/local/etc/php/conf.d/xdebug.ini`
And you just need to define build-time variable IP_ADDRESS during image building:
$ docker build --build-arg IP_ADDRESS=<IP_ADDRESS> .
If you use docker-compose:
1. Create file .env with the following content:
IP_ADDRESS="<IP_ADDRESS>"
You can make it every time like (example is for a linux machine):
IP_ADDRESS=$(ip a | grep <interface> | grep inet | awk '{print $2}' | awk -F'/' '{print $1}')
echo "IP_ADDRESS=$IP_ADDRESS" > .env
2. Use the following docker-compose.yaml to build your image:
version: '3'
services:
myservice:
build:
context: .
args:
IP_ADDRESS: ${IP_ADDRESS}
3. Build the above image:
docker-compose build

Answer 2

There's no simple in built way to get the Docker host IP (unless you are using Docker for Mac)
Entrypoint
It's best not to set a Docker host IP at build time, otherwise the image will be tied to the host it was built on and won't work anywhere else.
An ENTRYPOINT can be used to do the config setup based on an environment variable and then pass through all commands to the container:
#!/bin/sh
if [ -n "$IP_ADDRESS" ]; then
echo "xdebug.remote_host=$IP_ADDRESS" >> /usr/local/etc/php/conf.d/xdebug.ini
else
echo "No environment variable IP_ADDRESS set for xdebug"
fi
exec "$#"
Then run with:
docker run -e IP_ADDRESS=192.168.51.5 me/app-debug
Docker for Mac
On Docker for Mac 17.12+ you can use the host name docker.for.mac.host.internal
Xdebug
Another option is setting xdebug.remote_connect_back = 1 so you don't need a specific remote_host for xdebug.
Build
Nicolay's answer covers the build time setup.