Dockerfile: run go get in container creation

This significantly speeds up test execution

Signed-off-by: Adrian Moreno <amorenoz@redhat.com>

Dockerfile

@@ -6,7 +6,7 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
     libssl-dev  \
     llvm-dev \
     libjemalloc-dev \
-    libnuma-dev \
+    libnuma-dev  \
     python-sphinx \
     libelf-dev \
     selinux-policy-dev \
@@ -15,8 +15,15 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
     automake \
     libtool
 
-ADD .travis /src/travis
-WORKDIR /src/travis
+# Cache go mod dependencies to speed up test execution
+WORKDIR /src
+ADD go.mod /src/
+ADD go.sum /src/
+ENV GO111MODULE=on
+RUN go get -v ./...
 
+# Prepare the environment (builds ovsdb)
+WORKDIR /src/travis
+ADD .travis /src/travis
 ENV OVN_SRCDIR=/src/
 RUN sh ./test_prepare.sh