Layer `ADD source-code` prior to `make bootstrap` layer in Dockerfile (#3767)

Dockerfile

@@ -27,7 +27,7 @@ RUN git clone https://github.com/akesterson/shunit.git /tmp/shunit \
 WORKDIR /gopath/src/github.com/Azure/acs-engine
 
 # Cache vendor layer
-ADD Makefile test.mk versioning.mk Gopkg.toml Gopkg.lock /gopath/src/github.com/Azure/acs-engine/
+ADD . /gopath/src/github.com/Azure/acs-engine/
 RUN make bootstrap
 
 # https://github.com/dotnet/core/blob/master/release-notes/download-archives/2.1.2-sdk-download.md
@@ -46,4 +46,3 @@ RUN apt-get update \
     && apt-get update \
     && apt-get install azure-cli
 
-ADD . /gopath/src/github.com/Azure/acs-engine