docs: explain why Debian 9 is used as the base image; add some comman…

…d details to releasing.md

Dockerfile

@@ -1,5 +1,10 @@
 # Dfam TE Tools container including RepeatMasker, RepeatModeler, coseg
 
+# Why debian:9? glibc, and probably other libraries, occasionally take
+# advantage of new kernel syscalls. Docker containers normally run with the
+# host machine's kernel. Debian 9 has an old enough glibc to not have many
+# features that would only work on newer machines, and the other packages are
+# new enough to compile all of these dependencies.
 FROM debian:9 AS builder
 
 RUN apt-get -y update && apt-get -y install \

releasing.md

@@ -16,5 +16,7 @@
 4. Test the container
 5. Commit and tag in git as x.y
 6. Tag the container (in docker) as `:x`, `:x.y`, and `:latest`
+  * For each version, run: `docker image tag dfam/tetools:dev dfam/tetools:version`
 7. Push these tags to docker hub
+  * For each tag made before: `docker push dfam/tetools:version`
 8. Push the commit and tag to github