Turns out the problem was not in the environment variables after all...

I was able to check weather the settings.xml file was reading the env variables correctly by running:

mvn help:effective-settings

and noticed that they were being correctly parsed.

The actual problem was that, in my settings file, I only had the credentials set up for the snapshots directory and not for the central one.

Since I had changed the version name in the pom.xml to something that did not include -SNAPSHOT, maven was trying to deploy it on the central repo and was not able to find the credentials.

The issue was solved by adding:

<server>
   <id>central</id>
   <username>${env.M2_USERNAME}</username>
   <password>${env.M2_PASSWORD}</password>
</server>

to the settings.xml file

In the end there was nothing wrong with either docker nor maven. It was just my poor understanding of the maven build lifecycle :)