Docker: interesting features of basic images

When you specify in the Dockerfile:

 FROM ubuntu

then the officially-supported base image of Ubuntu is on your car (today it is 14.04 LTS). I wondered how this image differs from the “real” pure system.

In the process of pull you can see that it consists of 5 layers :

 docker pull ubuntu

 ubuntu: latest: The image you are pulling has been verified
 511136ea3c5a: Pull complete
 3b363fd9d7da: Downloading 8.641 MB / 197.2 MB 3m18s
 607c5d1cca71: Download complete
 f62feddc05dc: Download complete
 8eaa4ff06b53: Download complete

To understand the contents of each layer, you can run the command:

 docker history --no-trunc ubuntu > history.txt 

or see the original Dockerfile on github . Each layer corresponds to a separate line in the Dockerfile:
')
511136ea3c5a is the starting point, an empty image called scratch (line FROM scratch ).

3b363fd9d7da - “infusion” of the official Ubuntu image from the ubuntu-trusty-core-cloudimg-amd64-root.tar.gz file

Next comes the interesting.

607c5d1cca71 - a long chain of commands (it is recommended to combine teams in such a way as not to create a large number of layers, since this is fraught with problems ).

Let's stop here for a minute. First , the policy-rc.d script is added , which disables the automatic start of services. Details can be found in this article .

That is why you observe in the build process:

  invoke-rc.d: policy-rc.d denied execution of stop.
 invoke-rc.d: policy-rc.d denied execution of start. 

The next group of lines replaces the /sbin/initctl . The purpose of this change is to silence the warning when trying to execute the service command:

  Failed to connect to socket / com / ubuntu / upstart: Connection refused 

In upstart containers, of course, is not running (and in case of need, several services are recommended by Supervisor ).

This, in my opinion, is crooked: it would be easier to replace the initctl link to /bin/true , but the meaning does not change. dpkg-divert indicates that the initctl should not be overwritten during subsequent installations of the update.

The next step is configuring the package installer: disabling fsync to speed up (in case of problems you can always re-create the container from scratch), clearing the cache of deb files (it only takes up extra space in the layered file system), disabling installation of translations and activating index compression (not sure why it is necessary).

f62feddc05dc - for unknown reasons (probably only to insert a comment) , another command was removed from the previous thread. This line activates the universe package source, although it doesn’t do apt-get update , so before installing any universe package you will have to do the update yourself.

Finally, the last layer sets / bin / bash as the default command.

I thought that everyone who uses standard images would find it useful to know exactly what he is dealing with. That's all for now.