devtools/docker_ci.sh: upgrade to go 1.15
devtools/docker_ci.sh: upgrade to go 1.15

That is what our frontend runs on.

Change-Id: Iae80f7141f5a88f76567cce21a115b3385fbb2b5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/322009
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite