feat: add containerized build/test environment via Dockerfile

[alexeyqu]

Adds a Dockerfile pinned to gradle:8.10.2-jdk11 and a docker-compose.yml so contributors can run the full test suite with no local JDK installed:

docker compose run --rm test

The compose file encodes the source bind-mount and a named volume for the Gradle dependency cache, so the first run resolves dependencies and subsequent runs reuse them.

Notable decisions / tradeoffs

• Bind-mount design: source is bind-mounted at run time rather than COPYed into the image, so the image doesn't need rebuilding on every code change. The docker-compose.yml hides this from contributors -- they just run docker compose run.
• .gitignore cleanup: removed the bare Dockerfile rule. The rule's comment said it was for the demo-server's generated Dockerfile, but that file lands in distributions/demo-server/build/demo/Dockerfile -- already covered by the build/ ignore rule. The bare rule was redundant.
• Image name datatransferproject/dev: mirrors the existing datatransferproject/demo image produced by the :dockerize task, establishing a consistent namespace.
• JDK pinned to 11: the Gradle wrapper (6.9.2) can't parse Java 17 bytecode when compiling build scripts -- gradle:*-jdk17 fails outright. Not a preference; a hard constraint from the wrapper version.

[alexeyqu]

Looks like the Java CI failure is preexisting -- I reran the latest master branch and it failed despite passing last month -- investigating...

[alexeyqu]

Confirmed, looks like a flaky test -- see https://github.com/dtinit/data-transfer-project/actions/runs/26077383377 run history

[lisad]

additional documentation is appreciated!

[alexeyqu]

cc @simonxander -- I am curious if this direction would simplify your work / Developer Experience / deployment setup. Let me know if you'd be up for a discussion here (probably via a call).

[simonxander]

cc @simonxander -- I am curious if this direction would simplify your work / Developer Experience / deployment setup. Let me know if you'd be up for a discussion here (probably via a call).

Hi @alexeyqu, thanks for the ping!

I have tested the docker compose setup, and it works perfectly. This is very helpful for me, especially when switching between different projects that need different Java environments. It also makes setting up the environment much faster for new people joining the project.

For further discussion, I prefer text-based communication. Please feel free to share your thoughts right here in the comments!

[alexeyqu]

Thank you @simonxander -- I am continuing to improve the DevEx setup here, would greatly appreciate any ideas. Currently the next proposed step is some docker image tightening in #1505.