VOL-1985 Migrate voltha-simonu-adapter to go mod

Change-Id: I0dedc208a64e7478c4ef6adc0287430e7b7d6702
diff --git a/docker/Dockerfile.simulated_onu b/docker/Dockerfile.simulated_onu
index 26b3328..b9a346d 100644
--- a/docker/Dockerfile.simulated_onu
+++ b/docker/Dockerfile.simulated_onu
@@ -27,12 +27,10 @@
         protobuf-dev=3.6.1-r1
 
 # Prepare directory structure
-RUN mkdir -p /build \
-        "$GOPATH/src/" "$GOPATH/pkg" "$GOPATH/bin" \
-        "$GOPATH/src/github.com/opencord" \
-        "$GOPATH/src/github.com/opencord/voltha-simonu-adapter"
+RUN mkdir -p /build/src/simulated_onu \
+        "$GOPATH/src/" "$GOPATH/pkg" "$GOPATH/bin"
 
-WORKDIR $GOPATH/src/github.com/opencord/voltha-simonu-adapter
+WORKDIR /build/src/simulated_onu
 
 # Copy files
 COPY . .
@@ -45,9 +43,9 @@
 ARG org_opencord_vcs_dirty=unknown
 
 # Build
-WORKDIR $GOPATH/src/github.com/opencord/voltha-simonu-adapter
+WORKDIR /build/src/simulated_onu
 SHELL ["/bin/ash", "-o", "pipefail", "-c"]
-RUN go build -o /build/simulated_onu \
+RUN go build -mod=vendor -o /build/simulated_onu \
 	-ldflags \
 	"-X github.com/opencord/voltha-go/common/version.version=$org_label_schema_version \
 	 -X github.com/opencord/voltha-go/common/version.vcsRef=$org_label_schema_vcs_ref  \