This is useful for people who want to do multi-architecture Docker builds; it means the builder stage need only be run omce for all architectures, even though the final stage needs to be built for *each* architecture.
Change-Id: I2bccf5bcd21e37625477695ee927d49dd4333ac4
Change-Id: I6f60fae65f03740f26d94d9a3cc2fa2402511ca9