Title : Docker cheatsheet Author: Solène Date : 24 September 2020 Tags : docker Simple Docker cheatsheet. This is a short introduction about Docker usage and common questions I have been asking myself about Docker. The official documentation for building docker images can be [found here](https://docs.docker.com/develop/) ## Build an image Building an image is really easy. As a requirement, you need to be in a directory that can contain data you will use for building the image but most importantly, you need a `Dockerfile` file. The Dockerfile file hold all the instructions to create the container. A simple example would be this description: FROM busybox CMD "echo" "hello world" This will create a docker container using busybox base image and run `echo "hello world"` when you run it. To create the container, use the following command in the same directory in which Dockerfile is: $ docker build -t your-image-name . ## Advanced image building If you need to compile sources to distribute a working binary, you need to prepare the environment to have the required dependencies to compile and then you need to compile a static binary to ship the container without all the dependencies. In the following example we will use a debian environment to build the software downloaded by git. FROM debian as work WORKDIR /project RUN apt-get update RUN apt-get install -y git make gcc RUN git clone git://bitreich.org/sacc /project RUN apt-get install -y libncurses5-dev libncurses5 RUN make LDFLAGS="-static -lncurses -ltinfo" FROM debian COPY --from=work /project/sacc /usr/local/bin/sacc CMD "sacc" "gopherproject.org" I won't explain every command here, but you may see that I have split the packages installation in two commands. This was to help debugging. The trick here is that the docker build process has a cache feature. Every time you use a `FROM`, `COPY`, `RUN` or `CMD` docker will cache the current state of the build process, if you re-run the process docker will be able to pick up the most recent state until the change. I wasn't sure how to compile statically the software at first, and having to install git make and gcc and run git clone EVERY TIME was very time consuming and bandwidth consuming. In case you run this build and it fails, you can re-run the build and docker will catch up directly at the last working step. If you change a line, docker will reuse the last state with a FROM/COPY/RUN/CMD command before the changed line. Knowing about this is really important for more efficient cache use. ## Run an image With the previously locally built image we can run it with the command: $ docker run your-image-name hello world By default, when you use an image name to run, if you don't have a local image that match the name *docker* will check on the docker official repository if an image exists, if so, it will be pulled and run. $ docker run hello-world This is a sample official container that will display some explanations about docker. If you want to try a gopher client, I made a docker version of it that you can run with the following command: $ docker run -t -i rapennesolene/sacc Why did you require `-t` and `-i` parameters? The former is to tell docker you want a tty because it will manipulate a terminal and the latter is to ask an interactive session. ## Persistant data By default, every data of the docker container get wiped out once it stops, which may be really undesirable if you use docker to deploy a service that has a state and require an installation, configuration files etc... Docker has two ways to solve it: 1) map a local directory 2) map a docker volume name This is done with the parameter `-v` with the `docker run` command. $ docker run -v data:/var/www/html/ nextcloud This will map a persistent storage named "data" on the host on the path `/var/www/html` in the docker instance. By using `data`, docker will check if `/var/lib/docker/volumes/data` exists, if so it will reuse it and if not it will create it. This is a convenient way to name volumes and let docker manage it. The other way is to map a local path to a container environment path. $ docker run -v /home/nextcloud:/var/www/html nextcloud In this case, the directory `/home/nextcloud` on the host and `/var/www/html` in the docker environment will be the same directory.