make clean before build to avoid using binaries built outside of docker

Dockerfile

@@ -1,7 +1,7 @@
 FROM alpine:3.7 AS build
 RUN apk add --no-cache libgcc alpine-sdk gcc libpcap-dev linux-headers findutils
 COPY . /snabb
-RUN cd /snabb && make -j
+RUN cd /snabb && make clean && make -j
 
 FROM alpine:3.7
 RUN apk add --no-cache libgcc