add riscv64 build of docker-ce

[gdams]

- What I did

Added support for riscv64 to produce builds of docker-ce

- How I did it

Added riscv64 to common.mk file

- How to verify it

Test the output on a riscv64 host

- Description for the changelog

add initial release of riscv64 for docker-ce

- A picture of a cute animal (not mandatory but encouraged)