x/build: dockerize scaleway builders

[s-mang]

dockerize scaleway builders so we can start each build with a fresh container - same initial state for every build. the builder setup should mimic that of the linaro and packet builders.

@bradfitz anything else I should include?

[gopherbot]

CL https://golang.org/cl/43612 mentions this issue.

[gopherbot]

CL https://golang.org/cl/43813 mentions this issue.

[gopherbot]

CL https://golang.org/cl/44815 mentions this issue.