tests: fix docker image [skip ci]

tests/Dockerfile

@@ -1,7 +1,11 @@
 # stick to non-alpine so we can reuse build from travis host (avoid libc compat issues)
 FROM golang:1.12
 
-RUN apk update && apk add git make gcc
+ARG DEBIAN_FRONTEND=noninteractive
+
+RUN apt update \
+ && apt install -y git make gcc \
+ && rm -rf /var/lib/apt/lists/*
 RUN go get -d golang.zx2c4.com/wireguard \
  && cd /go/src/golang.zx2c4.com/wireguard \
  && rm donotuseon_linux.go \