fix: update Dockerfile and .dockerignore for SBT 2 stage path

[rabestro]

SBT 2 changed the output path for sbt-native-packager's stage task from target/universal/stage to target/out/jvm///universal/stage.

• Use dynamic path lookup in Dockerfile to find the stage directory
• Update .dockerignore to ignore target/ broadly and re-include target/out/

Fixes GitHub Actions build failure due to missing stage directory after migrating from SBT 1 to SBT 2.

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[gemini-code-assist]

Code Review

This pull request modifies the Docker build configuration by un-ignoring the target/out/ directory in .dockerignore and creating a symbolic link to the staged build directory in the Dockerfile. The review feedback correctly points out that the .dockerignore change is ineffective because the parent target/ directory is already excluded, and it advises against using a fragile find and ln -s combination in the Dockerfile, recommending a more robust cp command instead.

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[gemini-code-assist]

There are two issues with adding !target/out/ here:

1. Ineffectiveness: In .dockerignore, once a parent directory (like target/ on line 3) is excluded, Docker will not traverse into it to evaluate negation patterns. Therefore, !target/out/ has no effect as written.
2. Conceptual/Performance Issue: As noted in the comments on lines 1-2, this multi-stage build compiles from scratch inside the container. Uploading the host's target/out/ directory to the Docker daemon is unnecessary, increases the build context size, slows down the build, and can invalidate Docker's build cache.

Since the Dockerfile runs sbt stage to compile and stage the application from scratch, we should keep target/ completely ignored and remove this negation line.

[gemini-code-assist]

Using find with a shell variable and ln -s can be fragile. If find returns multiple paths (e.g., if multiple Scala versions or subprojects are built) or no paths at all, the ln -s command will fail.

Additionally, using symbolic links across multi-stage Docker builds can sometimes lead to issues during COPY --from if the symlink is not resolved correctly.

A more robust and simpler approach is to use standard shell wildcard expansion to copy the directory directly.

    GITHUB_TOKEN=$(cat /run/secrets/github_token) sbt stage && \
    mkdir -p /build/target/universal && \
    cp -a /build/target/out/jvm/*/*/universal/stage /build/target/universal/