Add crossbuild image flag

[SuperQ]

Make the crossbuild docker image a flag to allow local testing.

• Only pull the docker image if the default is used.

Signed-off-by: Ben Kochie [email protected]

[roidelapluie]

I think we could use that for more than local testing. You could add a flag to disable pulling instead ?

[roidelapluie]

Also the crossbuild image could be a .promu.yml thing. We could build prometheus with circleci-golang:1.16-node image.

[roidelapluie]

if you still need this .. I also see you have not answerd my questions?