Fix Dockerfile template variable names.

4 jobs for master in 3 minutes and 10 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #111508
build Rust 1.41 image with Alpine

00:00:53

passed #111509
build Rust 1.41 image with Debian

00:01:48

 
  Publish
passed #111510
tag Rust 1.41-alpine image

00:00:14

passed #111511
tag Rust 1.41-debian image

00:00:14