Links

Alpine S6

The base image for much of the bfren ecosystem, containing Alpine Linux installed with the S6 Overlay.
Source files (GitHub) Container images (Docker Hub)

Acknowledgements

The S6 Overlay is a version of the S6 supervisor which has various useful features for containers and managing services.

Environment Variables

Name
Values
Description
Default
BF_CRON_LOG_LEVEL
0-8
Cron log level (0 is most verbose).
8

Packages

The image comes with the S6 Overlay and all its dependencies.

Layout

The most important concept in these images is the /overlay/ directory, which is copied from the source over the top of the image. You can use whatever structure you like within that.
So, for example, the edge repositories are added via the /overlay/etc/apk/repositories file, which is copied over the top of the base image filesystem by the Dockerfile command COPY ./overlay /.
You can add any files you want to the image this way.

S6 Startup

Every time the container starts, the S6 Overlay runs the same series of scripts. These allow you to ensure that there is a consistent environment for your service.
The sequence is as follows (all should be contained within the /overlay/ directory):
  1. 1.
    /etc/fix-attrs.d/ - set required permissions
  2. 2.
    /etc/cont-init.d/ - run initialisation scripts
  3. 3.
    /etc/services.d/ - register the services you want to be supervised
My rule of thumb is that the first image uses 0-based files. Then the next uses 1-based files, then 2-based, etc. As S6 loads these files alphabetically, this practice ensures base permissions and configuration is always done first.
The S6 Overlay does more before and after all this - additional information can be found in their documentation. These three folders are where you will most often hook into the container processes.

Permissions

For me, permissions are one the most powerful and most frustrating features of Linux. These are set by adding files to /overlay/etc/fix-attrs.d/. Good practice is to prefix these files with a number so you have control over the order in which they are loaded. For example:
# fix-attrs.d/00-bin
/usr/local/bin/healthcheck false root:root 0500 0500
# fix-attrs.d/01-user
/etc/user true www:www 0644 0755
  • First we have the absolute path to the file or directory to set permissions for
  • Next we have true or false whether or not to recurse (obviously meaningless for a file)
  • Then we have the owner of the file or directory
  • Finally we have the permissions, first fmode (file permissions) and then dmode (directory permissions). I find the chmod calculator extremely useful for generating these.
You can have as many of these as you wish, and you can reapply them at any point by using bf-fix-attrs.

Initialisation

After the permissions are set, the scripts in /etc/cont-init.d/ are run. Here you might want to run installation routines, create configuration files, etc.
For example, in the ClamAV image the following two scripts run in this stage:/etc/cont-init.d/10-initial # downloads the latest virus definitions
/etc/cont-inid.d/11-updater # registers the freshclam update daemon
Notice that in this example the files begin 10- and 11- as mentioned earlier.
(It does mean there can't be more than ten initialisation scripts per image, but frankly if there are more we need to ask the question, is the image trying to do too much? A key principle of Docker is that an image should do one thing.)

Services

All the services you want to be managed by S6 go in /etc/services.d/. The name of the next directory will be the name of the service, e.g. cron. Within that the minimum you need is a file named run, which contains the script necessary to start your service.
Here is the run file from the cron service of this image:
#!/bin/sh
/usr/sbin/crond -f
As you can see, it doesn't have to be complex! The trick with this particular example is the -f flag which tells the cron daemon to run in the foreground. If it ran in the background, S6 wouldn't be able to supervise it. This is something to watch out for when creating your own services.
You can have as many services as you want - however the main point of Docker is to separate services into their own 'contained' environments. Therefore I try to keep the discipline of one 'main' service, and only adding 'supporting' services beyond that.
So, for example, in the Nginx image, the main service is nginx itself, but there are also two services to manage logging.
Note that service directories are not given a number prefix - they will run in any order. S6 does have ways of making a service wait for another service to start, but it's complex and very fiddly. Plus, if something needs to happen before your service starts, it should probably go in /etc/cont-init.d/ instead.
In your main service directory, I suggest also including a finish file:
#!/bin/sh
s6-svscanctl -t /var/run/s6/services
This is also from the cron service of this image, but you'll see it in the main service directory of all the bfren images. What this does is tell S6 to close down all the services gracefully when this one quits, and then stop the container. You don't have to do this, but it's good practice to ensure a safe closedown of your container services when one of them crashes.
It means you may want to set restart to unless-stopped instead of always, or you could end up with a never-ending loop of a container starting, crashing, restarting, crashing, etc.

Container Environment

One of the most useful tricks when using S6 is to make use of the following directive to import all environment variables into the current script:
#!/usr/bin/with-contenv sh
freshclam -d -c ${FRESHCLAM_PER_DAY}
This is an example from the ClamAV image (in fact it is the contents of the 11-updater file we looked at earlier). One of that image's environment variables is FRESHCLAM_PER_DAY which allows you to define how many times a day you want freshclam to run.
You do not get Docker's environment variables in scripts by default however, so you need to use the S6 helper function with-contenv (with container environment). Then you can access all the environment variables you want.

Have fun!

And that's it! There is a lot more to it if you want to get complicated - I suggest you read the S6 documentation if you want to go deeper..