Update templating to pin to only major version and use a single template

It's extremely unlikely that memcached is going to maintain more than one major version at a time (even that seems to be a bit much sometimes, which is relatable).

This also converts the Dockerfiles to use semicolons instead of `&&` chains (see https://github.com/docker-library/memcached/compare/b8a7264..00003b7 to compare this without that conversion).

Additionally, this converts us to just two variants and auto-detects the latest Alpine/Debian, since we don't have a good reason to support more than one of each Debian and Alpine.

Author: tianon

.gitattributes

@@ -0,0 +1,3 @@
+/*/**/Dockerfile            linguist-generated
+/*/**/docker-entrypoint.sh  linguist-generated
+/Dockerfile*.template       linguist-language=Dockerfile