Dockerfile: fix missing "boilerplate.txt" in build-stage
Dockerfile: fix missing "boilerplate.txt" in build-stage

Although the docs still rendered ok, some pieces were missing
when generating the "samples", which resulted in warnings during
build:

```
Adding front-matter to ./_samples/library/rethinkdb/README.md ...
cat: can't open './_samples/boilerplate.txt': No such file or directory
Adding front-matter to ./_samples/library/tomcat/README.md ...
cat: can't open './_samples/boilerplate.txt': No such file or directory
```

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

londoncalling/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io