docs: add alpine info (#654)

Signed-off-by: rare-magma <rare-magma@posteo.eu>

Author: rare-magma

doc_docker.md

@@ -2,6 +2,10 @@
 
 A docker version of LibreSpeed is available here: [GitHub Packages](https://github.com/librespeed/speedtest/pkgs/container/speedtest)
 
+# Alpine Linux variant
+
+An Alpine Linux based docker version of LibreSpeed is also available here: [GitHub Packages](https://github.com/librespeed/speedtest/pkgs/container/speedtest) under all the tags that have the `-alpine` suffix. This variant is significantly smaller but can have slightly different behaviour due to its toolchain being based in [musl](https://en.wikipedia.org/wiki/Musl) libc as mentioned in [here](https://alpinelinux.org/about/).
+
 ## Quickstart
 
 If you just want to try it, the fastest way is: