> wget https://releases.mattermost.com/focalboard/0.5.0/focalboard-...
> tar -xvzf focalboard-server-linux-amd64.tar.gz
The file is actually a zip with the tar.gz inside:
focalboard-server-linux-amd64.tar.gz: Zip archive data, at least v2.0 to extract
$ unzip -l focalboard-server-linux-amd64.tar.gz
Archive: focalboard-server-linux-amd64.tar.gz
Length Date Time Name
--------- ---------- ----- ----
12461347 02-02-2021 00:38 focalboard-server-linux-amd64.tar.gz
-------- -------
12461347 1 files
Running tar directly (as written in the instructions) will fail with:
$ tar -tzvf focalboard-server-linux-amd64.tar.gz
tar: invalid magic
tar: short read
It seems like there's something unintentionally producing a zip at the end of the build process.