Getting container's IP inside a dockerfile - reactjs

Question

I am running a reactjs app using docker container, and we are using Mock API and UI. I am running those inside a single docker container as 2 separate process. However, in the .env file of the reactjs app the environment variables are mapped to localhost like below :-
REACT_APP_MOCK_API_URL="http://localhost:8080/API"
REACT_APP_MOCK_API_URL_AUTH="http://localhost:8080/API/AUTH"
REACT_APP_MOCK_API_URL_PRESENTATION="http://localhost:8080/API/PRESENTATION"
Since the docker container's IP would be dynamic i need to override it with the dynamic ip that the container will be creating at run time.
May i know the way to do this inside dockerfile ???
PS : I tried assigning the static IP inside the docker file for these environment vars and it works. However, i am not sure how to get the IP dynamically and pass it inside the dockerfile itself .
Please help.
Thanks.

Answer 1

That's intrinsically not something you can directly set up inside the Dockerfile. You usually don't care about the container-internal IP addresses at all: from other containers you should use Docker's internal DNS service, and from outside a container you can access published ports (docker run -p option) via the host's IP address.
In many cases you can glean enough information from HTTP headers to construct valid links within an application. You might be able to set these variables to just e.g. REACT_APP_MOCK_API_URL="/API"; if that's interpreted relative to some other URL in the application then it will inherit the correct host name.
If none of this works, you can use an entrypoint script to set these variables. This might look something like:
#!/bin/sh
if [ -n "$URL_PREFIX" ]; then
# Set these three variables, if they're not already set
: ${REACT_APP_MOCK_API_URL:="${URL_PREFIX}/API"}
: ${REACT_APP_MOCK_API_URL_AUTH:="${URL_PREFIX}/API/AUTH"}
: ${REACT_APP_MOCK_API_URL_PRESENTATION:="${URL_PREFIX}/API/PRESENTATION"}
# Export them to other processes
export REACT_APP_MOCK_API_URL REACT_APP_MOCK_API_URL_AUTH
export REACT_APP_MOCK_API_URL_PRESENTATION
fi
# Launch the main container command
exec "$#"
In your Dockerfile you'd COPY this script in and run it as the ENTRYPOINT
...
COPY docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
CMD [...]
Then when you finally ran the container, you can dynamically inject the URL prefix, including whatever port you choose.
docker run -e URL_PREFIX="http://$(hostname):3456" -p 3456:8080 ...
The entrypoint script will set the other variables based on the URL_PREFIX variable, then run whatever command was set as the CMD in the Dockerfile or was named on the docker run command line. (If you docker run -it ... sh, the entrypoint will run and as its last step launch the interactive shell, which is useful for debugging.)