update docs

README.md

@@ -11,5 +11,10 @@ normally excluded) are available; run with -h to see what you can do.
 
 Now the script is containerized, and can be invoked with something like:
 
-```docker run -it --rm salfter/anytom4v -o video.m4v src_video.mkv```
+```docker run -it --rm -v /mnt/files:/mnt/files salfter/anytom4v -o /mnt/files/video.m4v /mnt/files/src_video.mkv```
 
+Development is over at https://gitlab.com/salfter/anytom4v-docker.
+
+**Update** (9 Jun 22): Switched the base container from Debian to Gentoo,
+as the script was developed on Gentoo and uses options that apparently
+aren't in Debian builds of ffmpeg.