fix: inline convert test Dockerfile data

[raharper]

Skip downloading Dockerfiles for the convert test as they add instability to the tests when github is unreliable. Capture what we are fetching and the sha256sum values.

What type of PR is this?

bug

Which issue does this PR fix:

Help removing instability in the convert test case

What does this PR do / Why do we need it:

We had a recent nightly CI failure due to github repo instability so instead of downloading (git clone, etc) capture the content in our repository.

If an issue # is not available please add repro steps and logs showing the issue:

https://github.com/project-stacker/stacker/actions/runs/23416513187/job/68112948797

# + sh -e -c curl --retry 10 -S -L --output /tmp/elasticsearch.tar.gz https://artifacts-no-kpi.elastic.co/downloads/elasticsearch/elasticsearch-8.17.10-linux-$(arch).tar.gz
#   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
#                                  Dload  Upload   Total   Spent    Left  Speed
# 
...
 84  462M   84  389M    0     0  13.4M      0  0:00:34  0:00:29  0:00:05 13.1M
# curl: (92) HTTP/2 stream 1 was not closed cleanly: INTERNAL_ERROR (err 2)

Testing done on this change:

CI

Automation added to e2e:

None

Will this break upgrades or downgrades?

No

Does this PR introduce any user-facing change?:

No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[raharper]

Looking inside the dockerfiles, this won't help since they fetch the same resources from github; they already have lots of retries...