Dockerfile.alpine: install Git & Mercurial

build/Dockerfile.alpine

@@ -3,6 +3,9 @@ FROM golang:1.13-alpine
 # required to support cgo
 RUN apk --no-cache add gcc musl-dev
 
+# needed most times for `go get`, etc. See https://github.com/docker-library/golang/issues/80
+RUN apk add --no-cache git mercurial
+
 # don't place it into $GOPATH/bin because Drone mounts $GOPATH as volume
 COPY golangci-lint /usr/bin/
 CMD ["golangci-lint"]