Heroku (Docker) PORT environment variable in nginx - docker

Question

I just take a few modifications on a Dockerfile to run it with nginx on Heroku. Something special about Heroku is, that everything is running as non-root. Second certain behaviour is the use of a random Port which comes from Heroku itself and you can't be modified. They provide the env $PORT which you should bind to nginx. If Heroku recognizes that something isn't bind to that port it stops the entire container. Question is:
How can I bind nginx to a given env variable in order to have a dynamic port in the nginx-site.conf?
I tried to use things like follows in the Dockefile:
env PORT; in nginx.conf and listen PORT_from_env; in the nginx-site.conf
Also tried listen 80; in the nginx-site.conf and RUN /bin/sed -i "s/listen 80/listen ${PORT}/" /etc/nginx/sites-available/default.conf in the Dockerfile
I am absolutely above my capabilities. Someone has an idea or can help? It be very helpful!
Further information:
https://devcenter.heroku.com/articles/dynos#web-dynos

Answer 1

I got it working for my app by following this example :
Step 1: listen to $PORT in default.conf.template
server {
listen $PORT default_server;
location / {
root /usr/share/nginx/html;
index index.html;
}
}
Step 2: add this directive to your Dockerfile
COPY default.conf.template /etc/nginx/conf.d/default.conf.template
Step 3: add this at the end of your Dockerfile
CMD /bin/bash -c "envsubst '\$PORT' < /etc/nginx/conf.d/default.conf.template > /etc/nginx/conf.d/default.conf" && nginx -g 'daemon off;'

Answer 2

I'll show a solution which doesn't require to write a new Dockerfile, you can use the offical nginx images.
Like #Jimmy's solution we'll use the envsubst command which substitutes environment variables in shell format strings.
This command is available with the offical nginx image and also with the alpine version.
Step #1
write your nginx configuration in a template file - let's call it: site.template:
server {
listen ${PORT};
server_name localhost;
location / {
root /usr/share/nginx/html;
index index.html index.htm;
}
}
Notice the PORT placeholder.
Step #2 - with docker compose
Mount that inside the /etc/nginx/conf.d directory and then execute the envsubst command to use the template as a reference for default.conf:
web:
image: nginx:alpine
volumes:
- ./site.template:/etc/nginx/conf.d/site.template
ports:
- "3000:8080"
environment:
- PORT=8080
command: /bin/sh -c "envsubst < /etc/nginx/conf.d/site.template > /etc/nginx/conf.d/default.conf && exec nginx -g 'daemon off;'"
Notice that:
1. You need to execute the nginx daemon after that.
2. I used /bin/sh and not /bin/bash because my base image is alpine.
Step #2 (Another option) - inline docker run
If, for some reason you don't want to work with docker-compose you can use the following bash script:
#!/usr/bin/env bash
##### Variables #####
PORT=8080 #Or $1 if you pass it from command line
TEMPLATE_DIR=$(pwd)/site.template
TEMPLATE_REMOTE_DIR=/etc/nginx/conf.d/site.template
IMAGE_NAME=nginx:alpine
echo "Starting nginx on port: $PORT ..."
##### The docker command #####
docker run -p 3000:$PORT -v $TEMPLATE_DIR:$TEMPLATE_REMOTE_DIR $IMAGE_NAME \
/bin/sh -c "envsubst < $TEMPLATE_REMOTE_DIR > /etc/nginx/conf.d/default.conf && exec nginx -g 'daemon off;'"

Answer 3

You need to update your nginx configuration at boot time.
See this buildpack as an example (I haven't tested that it does work).