This docker file's goal is to:
Goal: provide a thrift-compiler Docker image
I was just wondering why does this image need to install golang
It appears to download the Golang binary package but only copies over gofmt. Looking at https://github.com/apache/thrift/blob/19baeefd8c38d62085891d7956349601f79448b3/compiler/cpp/src/thrift/generate/t_go_generator.cc it seems that at one point they were running gofmt on the Golang generated code.
The comment for that part of code links to https://issues.apache.org/jira/browse/THRIFT-3893 which references pull request https://github.com/apache/thrift/pull/1061 where the feature was actually removed.
The specific commit (https://github.com/apache/thrift/commit/2007783e874d524a46b818598a45078448ecc53e) appears to be in 0.10 but not 0.9. So, along with the disabling of gofmt they probably just forgot to remove it from the image or decided it was just worth leaving as the feature could be fixed and re-enabled at a later date.
It might be worth opening an issue to ask the Thrift team about it and if it can be removed.