VyOS in Docker: T2640: Fixed builds for sagitta

[zdc]

Change Summary

Fixed the problem when APT repositories were not configured properly in VyOS 1.4 / sagitta during a Docker image build.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://phabricator.vyos.net/T2640

Component(s) name

VyOS Docker image

Proposed changes

After creating the new release train sagitta the script that configures the APT repository for a Docker image was not able to do this properly, due to missing this train in the if condition.
Also, fixed the bug when multiple APT preferences files lead to the wrong syntax in a preference file inside a Docker container.

How to test

Run this before and after the commit will produce broken and normal VyOS images accordingly.

git clone -b current https://github.com/vyos/vyos-build.git
cd vyos-build/docker-vyos/
docker build --no-cache --pull --compress -f Dockerfile -t vyos:1.4-`date -u +%Y%m%d%H%M%S` --build-arg BUILD_DATE="`date -u --rfc-3339=seconds`" --build-arg VYOS_VERSION=1.4 --build-arg DEBIAN_VERSION=buster --progress plain ..

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly