Fix Infra-PerCommit-CreateDockerImage.
Fix Infra-PerCommit-CreateDockerImage.

This was broken in https://skia-review.googlesource.com/c/buildbot/+/329087 by introducing a reference to Regexp.SubexpIndex[1], which is only available in Go >1.15. Our repo's Go version is pinned at 1.13 as per our go.mod file.

[1] https://golang.org/pkg/regexp/#Regexp.SubexpIndex

Change-Id: Ie7787db1322c52f70f1c17a7c9b4481ac5d3d86c
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/329707
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot