Docker advanced topics


Teaching: 15 min
Exercises: 0 min
  • Learn how to set custom user and group IDs in a container

  • Learn how to redirect input to a container

  • Learn how to use Docker Compose to manage multiple containers for web services

Input redirection with Docker

Suppose you need to redirect input to a Docker container, either using an input file with < or piping from another execution with |. If you just run the container as usual you won’t get the expected result. As an example, let’s use the unix command wc -w to count the number of words received in input; we’ll produce the words with echo:

$ echo "one two three" | docker run ubuntu wc -w

We are getting 0 instead of 3, suggesting input redirection is not happening. To make it work, we need to use the additional flag -i, which keeps Docker STDIN (standard input) open:

$ echo "one two three" | docker run -i ubuntu wc -w

Here we go!

The same flag is required when receiving input from a file, for instance let’s create a file with some words:

$ echo "one two three" > words

and then try and count them:

$ docker run ubuntu wc -w < words
$ docker run -i ubuntu wc -w < words

Matching user permissions with the host

Try and create an empty file and then see who is the owner:

$ docker run -v `pwd`:/data -w /data ubuntu touch owner1
$ ls -l owner1
-rw-r--r-- 1 root   root   0 Jul  3 02:11 owner1

So, as by default a container is run as root, any created file belongs to the root user! This can be annoying, in that the host user might then have limitations in editing/deleting those files.

Docker has an option, -u or --user, to alter the user and group ID in the running container. It can be used in conjunction with the linux command id to pass the host user and group IDs directly to the container. For instance:

$ docker run -v `pwd`:/data -w /data -u `id -u`:`id -g` ubuntu touch owner2 

Now, let us inspect the ownerships of all the files created so far through containers:

$ ls -l owner?
-rw-r--r-- 1 root   root   0 Jul  3 02:11 owner1
-rw-r--r-- 1 ubuntu ubuntu 0 Jul  3 02:11 owner2

As desired, the last created file is owned by the host user.

When running a container interactively with host IDs, you might get warnings of this type:

$ docker run -it -u `id -u`:`id -g` ubuntu bash
groups: cannot find name for group ID 1000
I have no name!@9bfdf83aed93:/data$

These can typically be ignored.

I have no name!@9bfdf83aed93:/data$ exit   # or hit CTRL-D

Finally, third-party containers might have been set-up so that permissions of standard users are more restricted compared to root. There are some critical cases here: only root can execute the application, or only root can write on certain locations that need to be modified at runtime. In these cases, the typical solutions are:

Long running services with Docker Compose

Sometimes we may need to run and manage multiple containers (e.g., running an Nginx web-server in front of some other application we are running in a container).

We can run all of these via docker run commands, but it may get cumbersome if you have lots of arguments and flags. We can use a tool called docker-compose to orchestrate and manage multiple containers for us.

All we need to do is define the various options and properties for our containers in a YAML file. Let’s create a simple, containerised MySQL/Nginx setup:

version: '3'

  # Nginx
    image: nginx:alpine
    container_name: nginx-webserver
    restart: unless-stopped
    tty: true
      - "80:80"
      - "443:443"

  # MySQL
    image: mysql:5.7.22
    container_name: db
    restart: unless-stopped
    tty: true
      - "3306:3306"
      MYSQL_DATABASE: mydb
      MYSQL_ROOT_PASSWORD: password
      - <PICK-A-HOST-DIRECTORY>:/var/lib/mysql

Here we can define different services and options. There are a lot of options, so I’ll touch on a few:

To run this, you simply need to save the above file as docker-compose.yml, cd to that directory and run:

$ docker-compose up -d

To shut it down, from the same directory run:

$ docker-compose down

Another example of usage for docker-compose can be found at this Github repo:

The YAML file in this repo sets up a secure RStudio server using Nginx.

Key Points

  • Change user that runs the container with the flag -u <user>:<group>

  • Make the container accept input from STDIN with the flag -i

  • You can use a Docker Compose YAML file to orchestrate the setup of multiple containers at once