Zstd compress all images

I tried gzip, bzip2, xz and xzstd. Xz 22s for a 128MB disk image. Out of
all other options zstd was both the fastest to compress (<1s) and
produced the smallest file (28MB).

.gitlab-ci.yml

@@ -21,7 +21,8 @@ before_script:
         nasm \
         pkg-config \
         texinfo \
-        wget &&
+        wget \
+        zstd &&
     curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain none
 
 img:

mk/ci.mk

@@ -8,6 +8,7 @@ ci-img: FORCE
 	rm -rf $(IMG_DIR)
 	mkdir -p $(IMG_DIR)
 	$(MAKE) demo desktop server
+	cd $(IMG_DIR) && zstd --rm *
 	cd $(IMG_DIR) && sha256sum -b * > SHA256SUM
 
 # The name of the target must match the name of the filesystem config file