unix/linux: update Dockerfile to Go 1.15.4
unix/linux: update Dockerfile to Go 1.15.4

No changes in generated files.

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>
Change-Id: Ibbb81f8ef48d43d7fb620fec57c3ac779ebf0e11
Reviewed-on: https://go-review.googlesource.com/c/sys/+/268357
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

golang/sys,golang/sys,golang/sys