build: add trailing newline when patching Dockerfile

[frasertweedale]

When check-container-build-with-prci.sh patches the Dockerfile, it does not append a trailing newline to the new COPY line. If there is an empty line following the FROM directive, this is fine. But otherwise, it breaks the Dockerfile.

Append a trailing newline so that adjacent lines do not get joined by accident.

[avisiedo]

LGTM

[frasertweedale]

Thanks @avisiedo.