VOL-1984 Migrate voltha-simolt-adapter to go mod

Change-Id: I8c167fb179b699b9cea02e36c20e1d28db61f41a
diff --git a/docker/Dockerfile.simulated_olt b/docker/Dockerfile.simulated_olt
index 4ee142a..7346bc9 100644
--- a/docker/Dockerfile.simulated_olt
+++ b/docker/Dockerfile.simulated_olt
@@ -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" \
-        "$GOPATH/src/github.com/opencord/voltha-simolt-adapter"
+RUN mkdir -p /build/src/simulated_olt \
+        "$GOPATH/src/" "$GOPATH/pkg" "$GOPATH/bin"
 
-WORKDIR $GOPATH/src/github.com/opencord/voltha-simolt-adapter
+WORKDIR /build/src/simulated_olt
 
 # Copy files
 COPY . .
@@ -45,9 +43,9 @@
 ARG org_opencord_vcs_dirty=unknown
 
 # Build
-WORKDIR $GOPATH/src/github.com/opencord/voltha-simolt-adapter
+WORKDIR /build/src/simulated_olt
 SHELL ["/bin/ash", "-o", "pipefail", "-c"]
-RUN go build -o /build/simulated_olt \
+RUN go build -mod=vendor -o /build/simulated_olt \
 	-ldflags \
 	"-X github.com/opencord/voltha-go/common/version.version=$org_label_schema_version \
 	 -X github.com/opencord/voltha-go/common/version.vcsRef=$org_label_schema_vcs_ref  \