Add missing zstd-libs to final Docker image

[BuckarooBanzay]

The generated docker image does not currently work because of a missing zstd dependency in the final layer.
It fails since #10788

Error message:

Error loading shared library libzstd.so.1: No such file or directory (needed by /usr/local/bin/minetestserver)
Error relocating /usr/local/bin/minetestserver: ZSTD_initCStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_compressStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_freeCStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_createDStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_decompressStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_initDStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_createCStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_isError: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_freeDStream: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_getErrorName: symbol not found
Error relocating /usr/local/bin/minetestserver: ZSTD_endStream: symbol not found

This PR adds:

• the zstd-libs to the final layer
• the minetestserver --version command after the docker build to verify it

How to test

docker build . -t minetest
docker run --rm -it minetest

The usual minetest banner should pop up afterwards

[sfan5]

Can you add a command that runs minetestserver --version inside the built Docker container (is that possible?) to the CI job so this doesn't happen again?

[BuckarooBanzay]

Can you add a command that runs minetestserver --version inside the built Docker container (is that possible?) to the CI job so this doesn't happen again?

done 👍 seems to work