Docker/Compose: Define and use env variable - apache

Question

on my github i'm creating a little fork from a debian minimal docker image. Its actually 5 packages which build up on previous:
debian-base-minimal
debian-base-standard
debian-base-security
debian-base-apache
debian-base-apache-php
On debian-base-apache i want to get a working env variable, which i can define later in docker-compose file. What should the env do?
Its should, if defined over docker-compose, write ServerName $SERVER_NAME at the end of /etc/apache2/apache2.conf to set a globally Server Name. If empty, no new line should be written.
But why its should write nothing when its empty? Cauz on build the Dockerfile to an image shouldnt include the SERVER_NAME.
I already tried something like:
echo "ServerName $SERVER_NAME" >> /etc/apache2/apache2.conf
on my 040-debian-base-apache file. But on build its wrote ServerName in, cauz i didnt defined a value and its using null. If i set a default in Dockerfile (ENV SERVER_NAME=127.0.0.1) its build the image with 127.0.0.1 and i cant change 127.0.0.1 via variable, cauz the variable already filled in with the value.
On ouput of the building with defined ENV SERVER_NAME=127.0.0.1 in Dockerfile (actually not in repo):
[...]
+ echo 'ServerName 127.0.0.1'
+ /etc/init.d/apache2 stop
Stopping Apache httpd web server: apache2.
ok.
+ /etc/init.d/apache2 start
Starting Apache httpd web server: apache2.
ok.
[...]
Its would be okay, if there stands default 127.0.0.1 cauz the apache can start. But i cant define it now in docker-compose.yml cauz its hardcoded 127.0.0.1 and not the output of a variable.
On ouput of the building with none defined ENV in Dockerfile (actually repo version):
[...]
+ echo 'ServerName '
+ /etc/init.d/apache2 stop
Stopping Apache httpd web server: apache2.
+ /etc/init.d/apache2 start
Starting Apache httpd web server: apache2 failed!
The apache2 configtest failed. ... (warning).
Output of config test was:
AH00526: Syntax error on line 228 of /etc/apache2/apache2.conf:
ServerName takes one argument, The hostname and port of the server
[...]
Can anybody help me to get this working? Would be nice to understand how it works.
Many thanks in advance.

Answer 1

As you've observed, every RUN command in the Dockerfile happens at docker build time, and in particular the contents of the file will be fixed based on what the environment variable was when you ran the build. You want it to change based on the runtime value of the variable, which means you need to write a script that runs at startup to do this.
A typical approach is to write an ENTRYPOINT script that does the first-time setup. The ENTRYPOINT gets passed the CMD (or whatever command got passed into docker run) as command-line arguments, so if it ends with exec "$#", the last thing it does is launch the "normal" command. You can use any ordinary shell script logic here, so I might write
#!/bin/sh
if [ -n "$SERVER_NAME" ]; then
echo "ServerName $SERVER_NAME" >> /etc/apache2/apache2.conf
fi
exec "$#"
Then you can provide this in your Dockerfile
COPY entrypoint.sh /
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]
CMD ["apachectl", "-DFOREGROUND"]
(The chmod isn't necessary if you can guarantee the file has execute permissions on your non-Windows host. The ENTRYPOINT must use the JSON-ish form. If you have another image that builds on top of this, remember that the combined image gets only one ENTRYPOINT and one CMD; the very deep stack of images you suggest is a pretty unusual setup.)