docker: build with explicit go version

It is occasionally useful to assert that our output was built with >=x
version of the Go compiler or linked against >=x version of the Go
standard library.  While it is possible to make this assertion after the
fact -- say, by dumping the value of runtime.buildVersion with a
debugger -- it's usually much easier to inspect this file.

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:alpine AS builder
+FROM golang:1.15.5-alpine3.12 AS builder
 
 RUN apk -v --no-progress --no-cache add git