.dockerignore: add

I'm tired of sketch committing .DS_Store files.

There's probably a more general fix to be done here
that helps all users, but I don't see its shape,
and this at least eases my frustration in the near term.

Co-Authored-By: sketch <hello@sketch.dev>
diff --git a/.dockerignore b/.dockerignore
new file mode 100644
index 0000000..4f074fd
--- /dev/null
+++ b/.dockerignore
@@ -0,0 +1,2 @@
+.DS_Store
+**/.DS_Store