Confusion while deploying docker-composer image - ruby-on-rails

Question

I've been working in a sample ruby-on-rails application and deploying docker image in a linux server (ubuntu 14.04).
Here is my Dockerfile:
FROM ruby:2.1.5
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
RUN mkdir /rails_docker_demo
WORKDIR /rails_docker_demo
ADD Gemfile /rails_docker_demo/Gemfile
ADD Gemfile.lock /rails_docker_demo/Gemfile.lock
RUN bundle install
ADD . /rails_docker_demo
# CMD bundle exec rails s -p 3000 -b 0.0.0.0
# EXPOSE 3000
docker-compose.yml:
version: '2'
services:
db:
image: postgres
web:
build: .
command: bundle exec rails s -p 3000 -b '0.0.0.0'
image: atulkhanduri/rails_docker_demos
volumes:
- .:/rails_docker_demo
ports:
- "3000:3000"
depends_on:
- db
deploy.sh:
#!/bin/bash
docker build -t atulkhanduri/rails_docker_demo .
docker push atulkhanduri/rails_docker_demo
ssh username#ip-address << EOF
docker pull atulkhanduri/rails_docker_demo:latest
docker stop web || true
docker rm web || true
docker rmi atulkhanduri/rails_docker_demo:current || true
docker tag atulkhanduri/rails_docker_demo:latest atulkhanduri/rails_docker_demo:current
docker run -d --restart always --name web -p 3000:3000 atulkhanduri/rails_docker_demo:current
EOF
Now my problem is that I'm not able to use docker-compose commands like docker-compose up, to run the application server.
When I uncomment the last two lines fromDockerfile i.e,
CMD bundle exec rails s -p 3000 -b 0.0.0.0
EXPOSE 3000
then I'm able to run the server on port 3000 but getting error could not translate host name "db" to address: Name or service not known. (my database.yml has "db" as host.) This is because postgres image is not used as I'm not using docker-compose file is not.
EDIT:
Output of docker network ls:
NETWORK ID NAME DRIVER SCOPE
b466c9f566a4 bridge bridge local
7cce2e53ee5b host host local
bfa28a6fe173 none null local
P.S: I've searched a lot in the internet but not yet able to use the docker-compose file.

Answer 1

Assumptions
If I am reading what you've done here correctly, my answer assumes the following two things.
You are using docker-compose to run the database container.
You are using plain docker commands (not docker-compose) to start the application server ("web").
First, I would suggest not doing that, it is a lot simpler to use docker-compose for both. However, I'll answer based on the above, assuming that there is some valid reason you cannot use docker-compose to run the "web" container.
About container and network names
When you run the docker-compose command to start the db container, among other things, two things happen.
The container is given a new name, composed of the directory you run the compose setup from, the static name in compose (db), and a number. So let's say you have this all in a directory name myapp, you would have a new container named myapp_db_1. You can see what it is named using docker ps.
A network bridge is created if it didn't already exist, named something like myapp_default - again, named after the directory that the compose setup is inside of.
Connecting to the right network
The problem is that your non-compose container is attached to the default network (probably docker_default), but your db container is attached to myapp_default. The two networks do not know about each other. You need to connect them. It probably makes more sense to tell the web app container to attach to the compose network.
First, get the correct network name. You can see all networks using docker network ls. It might look like this:
$ docker network ls
NETWORK ID NAME DRIVER SCOPE
c1f5764a112b bridge bridge local
175efb89adef docker_default bridge local
5185ff0e1054 myapp_default bridge local
Once you have the correct name, update your run command to know about the network using the --network option.
docker run -d --restart always --name web \
-p 3000:3000 --network myapp_default \
atulkhanduri/rails_docker_demo:current
Once it is attached to the proper network, the name "db" should resolve correctly.
If you used docker-compose to start both of them, this would not be necessary (this is one of the things docker-compose just takes care of for you silently).
Getting this to run on your server
In the comments, you mention that you are having some issues with compose on the server. Specifically you said:
Do I need to copy my complete project on the server? Can't I run the application from docker image only? Actually, I've copied docker-compose in server and it throws errors for Gemfile, then I copied Gemfile, then it says it should be a rails app. So I guess I need to copy my complete folder in server. Can you please confirm?
Let's look at some parts of your Dockerfile. I'll add some comments inline.
## Make a new directory, and then make it the current directory
RUN mkdir /rails_docker_demo
WORKDIR /rails_docker_demo
## Copy Gemfile and Gemfile.lock into this directory from outside
ADD Gemfile /rails_docker_demo/Gemfile
ADD Gemfile.lock /rails_docker_demo/Gemfile.lock
## Run the bundle installer, which will install to this directory
RUN bundle install
## Finally, copy everything from the outside local dir to here
ADD . /rails_docker_demo
So, clearly, /rails_docker_demo is your application directory within the container. You've installed a bunch of stuff here, and this will become a part of your image. When you push your image to the registry, then pull it down on the server (as you do in the deploy script), this will all come with it.
Now let's look at (some of) docker-compose.yml.
services:
web:
volumes:
- .:/rails_docker_demo
Here you have defined a volume mount, mounting the current directory (wherever docker-compose.yml lives) as /rails_docker_demo. When you do that, whatever happens to exist on the server is now available in /rails_docker_demo, but this mount undoes all the work from Dockerfile that I just mentioned above. Instead of having the resources you installed when you built the image, you have only whatever is on the server in the . directory. The mount is on top of the image's existing /rails_docker_demo directory, hiding its contents and replacing them with whatever is on the server at the moment.
Unless there is a reason you put this mount here, you probably just need to remove that volume mount from docker-compose.yml. You will still need docker-compose.yml on the server, but you should not need the rest of it (aside from the image, of course).
This mount you have done is a useful thing - for development purposes. It would let you use the container to run the application and quickly have code changes show up (without rebuilding the image). But in the case of your deployment, it is just causing trouble.

Answer 2

Try moving the EXPOSE above CMD, .e.g.
FROM ruby:2.1.5
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
RUN mkdir /rails_docker_demo
WORKDIR /rails_docker_demo
ADD Gemfile /rails_docker_demo/Gemfile
ADD Gemfile.lock /rails_docker_demo/Gemfile.lock
RUN bundle install
ADD . /rails_docker_demo
EXPOSE 3000
CMD bundle exec rails s -p 3000 -b 0.0.0.0