build-tools: support building with container (#2035)
build-tools: support building with container (#2035)

In container based builds, without `--load` or `--push`, building goes
to /dev/null. Update to always use `--load` (`--push` is not ideal since
we have some DRY_RUN logic, merging manifests, etc that make it tricky).
istio/tools,istio/tools,istio/tools,istio/tools,istio/tools,istio/tools