dist/tools/buildsys_sanity_check: use release version for Docker check

[crasbe]

Contribution description

Currently, the docker image version is static and is not updated when a release happens.
If the Docker container is updated in the meantime, no backports to the release branch are possible anymore because the static test fails.

Therefore, I extended the buildsystem sanity check to check if a VERSION file is present in RIOTBASE and if it is, the version string from that file is used to fetch the hash from docker.io to compare to the DOCKER_TESTED_IMAGE_REPO_DIGEST variable in makefiles/docker.inc.mk instead of getting the latest hash.

Testing procedure

The test should still succeed on master:

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ ./dist/tools/buildsystem_sanity_check/check.sh
cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ echo $?
0

The test should also succeed on a release branch. In our current situation, this will only work on the 2026.07-branch because the Docker image happens to be at 2026.07 and hasn't been updated as thoroughly as it should've been before.
This has been addressed in the release guides though in #22214, however I might have to extend the documentation there a little bit as well as in the docker container itself.
(Added blank lines to the logs to make it easier to read).

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ git checkout 2026.07-branch
Switched to branch '2026.07-branch'
Your branch is up to date with 'origin/2026.07-branch'.

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ git cherry-pick 2f99df8777
[2026.07-branch 7e1d25c579] dist/tools/buildsys_sanity_check: use release version for Docker check
 Date: Tue Aug 18 12:57:37 2026 +0200
 2 files changed, 15 insertions(+), 3 deletions(-)

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ echo "RIOT_VERSION = 2026.07" > VERSION

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ cat VERSION
RIOT_VERSION = 2026.07

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ ./dist/tools/buildsystem_sanity_check/check.sh

cbuec@W11nMate:~/RIOTstuff/riot-vanilla/RIOT$ echo $?
0

Issues/PRs references

Required for RIOT-OS/riotdocker#64 and RIOT-OS/riotdocker#287

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• Claude to make the change in the shell script.

[riot-ci]

Murdock results

✔️ PASSED

2f99df8 dist/tools/buildsys_sanity_check: use release version for Docker check

Success	Failures	Total	Runtime
1	0	1	01m:52s

Artifacts

• Documentation preview

[AnnsAnns]

trust