[BIGTOP-3899] Exclude docker/bigtop-slaves/Dockerfile from git

[pan3793]

Description of PR

Exclude the temp file docker/bigtop-slaves/Dockerfile for git which is created during ./gradlew -Pprefix=3.2.0 bigtop-slaves, and if the task failed, the file stays here.

How was this patch tested?

Local command line.

For code changes:

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