Set the LANG env variable for our images

It's not clear whether this will fix any existing issues, but it does seem like a good idea.

Details on the locale alpine ships with can be found here: https://wiki.musl-libc.org/functional-differences-from-glibc.html#Character_sets_and_locale

Here's the upstream issue for defaulting this in alpine: https://bugs.alpinelinux.org/issues/7374

And here is an example of another image doing this, the openjdk jre alpine image: https://github.com/docker-library/openjdk/blob/2598f7123fce9ea870e67f8f9df745b2b49866c0/8-jre/alpine/Dockerfile#L20