Update golang:1.15-alpine Docker digest to 86a9ac8

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	Sourcegraph
golang	stage	digest	1.15-alpine

---

Renovate configuration

Schedule: "on the 1st through 7th day of the month" in timezone America/Los_Angeles.

Automerge: Disabled by config. Please merge this manually once you are satisfied.

Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

Ignore: Close this PR and you won't be reminded about this update again.

---

• If you want to rebase/retry this PR, check this box

---

This PR has been generated by WhiteSource Renovate. View repository job log here.