tweak(file-templates): update dockerfile template

- Preserves file indentation
- Replace deprecated `MAINTAINER` instruction with `LABEL`

Ref: https://docs.docker.com/reference/dockerfile/#maintainer-deprecated

modules/editor/file-templates/templates/dockerfile-mode/__

@@ -1,4 +1,6 @@
+# expand-env: ((yas-indent-line 'fixed) (yas-wrap-around-region 'nil))
+# --
 FROM ${1:phusion/baseimage:latest}
-MAINTAINER ${2:`user-full-name` <`user-mail-address`>}
+LABEL org.opencontainers.image.authors="${2:`user-full-name` <`user-mail-address`>}"
 
 $0