Make max_size mandatory and remove default value

[jhchabran]

Follow-up to #654 (comment)

This PR makes the --max_size flag required and removes the default value, making the default behaviour and configuration more predictable.

@mostynb I couldn't test the :bazel-remote-image and its arm64 variant on my machine, seems it finds no matching toolchain on my laptop (darwin/arm64), meaning I haven't been able to test how to pass the size flag to bazel run :bazel-remote-image to update the README.

If you tell me what's missing, I'd be happy to fix that.

---

~/work/bazel-remote  rework-max_size-defaults $ bazel run :bazel-remote-image                                         
ERROR: /private/var/tmp/_bazel_tech/67a5c8228e3eb4092fc57f363bbb18b5/external/io_bazel_rules_go/BUILD.bazel:86:17: While resolving toolchains for target @io_bazel_rules_go//:cgo_context_data: No matching toolchains fou
nd for types @bazel_tools//tools/cpp:toolchain_type.
To debug, rerun with --toolchain_resolution_debug='@bazel_tools//tools/cpp:toolchain_type'
If platforms or toolchains are a new concept for you, we'd encourage reading https://bazel.build/concepts/platforms-intro.
ERROR: Analysis of target '//:bazel-remote-image' failed; build aborted: 
INFO: Elapsed time: 0.188s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded, 0 targets configured)
ERROR: Build failed. Not running target

[mostynb]

Re testing this on arm mac, you can try this:
bazel build //:bazel-remote-darwin-arm64

I made some changes recently to make sure that cgo is enabled, and there are a few sharp edges left.

[jhchabran]

Re testing this on arm mac, you can try this: bazel build //:bazel-remote-darwin-arm64

I made some changes recently to make sure that cgo is enabled, and there are a few sharp edges left.

Thanks, it worked. I also update the bazel run ... examples so they come with the flags.

[mostynb]

I made a few tiny tweaks and landed this change on the master branch- thanks again for the contribution.