Customise a configuration file from Dockerfile

Question

I am trying to create a postfix container.

What I want to do is to send emails from another container.

Here is what I've done:

FROM ubuntu:latest

RUN apt-get update && apt install -y postfix

COPY main.cf /etc/postfix/main.cf
COPY master.cf /etc/postfix/master.cf
COPY mynetworks /etc/postfix/mynetworks
COPY mydestinations /etc/postfix/mydestinations

CMD service postfix restart && tail -f /dev/null

It works but I want to improve it.

• I don't want any external configuration files dependencies: master.cf and main.cf. Is there a way, to say, in a Dockerfile: remove this line from main.cf and add this one instead. I have tried to do it with bash script but it is ugly. Is there a Dockerfile command to say: "I want to set this variable in this configuration file with this value" ?

• I need to put my second docker container IP in /etc/postfix/mynetworks file. But this IP address may change other time. I have tried to put its hostname in /etc/postfix/mynetworks but it doesn't work. How can I do ?

Thanks

Answer 1

Replacing lines in a file directly from Dockerfile

For the lines that exist and should be replaced you could invoke sed with RUN, e.g.,

RUN sed -i \
    "s|^mynetworks\s=.*$|mynetworks = cidr:/etc/postfix/mynetworks|" \ 
    /etc/postfix/main.cf

Use another authentication than IP address

If you would have to put dynamic IP addresses in your mynetworks and you cannot trust an entire CIDR subnet, you are really using a poor authentication method for the purpose. Instead of trusting IP addresses, you should use, e.g., password authentication (SASL) or client certificates.