install: Detect Ubuntu codename without /etc/lsb-release

[vvoland]

Ubuntu 26.10 (stonking) images no longer ship /etc/lsb-release, and
the base image has no lsb_release command.
Both codename lookups returned nothing, so the apt source line had no
suite and apt-get update failed:

deb [...] https://download-stage.docker.com/linux/ubuntu  test
E: Malformed entry 1 in list file /etc/apt/sources.list.d/docker.list (Component)

Read VERSION_CODENAME from /etc/os-release first, which every supported
Ubuntu release provides and get_distribution already relies on.
lsb_release and /etc/lsb-release remain as fallbacks when os-release has
no codename.

[thaJeztah]

Was curious of course if it's (e.g.) now an optional install or deprecated, but seems related to https://ubuntuhandbook.org/index.php/2026/07/ubuntu-remove-etc-debian-version/

As the Ubuntu developer announced, scripts and programs should look at /etc/os-release file, which has been the de-facto standard in the last 10 years, instead of /etc/debian_version. And, due to the favor of /etc/os-release, most recent build of base-files has removed /etc/lsb-release.

• /etc/lsb-release: Ubuntu 26.10 has removed this legacy file from base-files, in favor of /etc/os-release. This is an intentional migration; https://ubuntuhandbook.org/index.php/2026/07/ubuntu-remove-etc-debian-version/
• lsb_release command: Still available through the optional lsb-release package in Ubuntu 26.10. It now uses a minimal POSIX shell implementation that reads /etc/os-release, rather than requiring Python or the original LSB implementation.

[thaJeztah]

LGTM