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
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.