Implement "strict mode" which requires ko:// prefix for import paths

[imjasonh]

Demonstrating a strawman implementation and UX for "strict mode", as described in #9

In strict mode, image references are only considered if they begin with the string ko://. In loose mode, ko:// refs are considered as image references and built and replaced during YAML resolution.

If we decide to pursue this model, we could start by making strictness opt-in, cut a versioned release of ko, then make --strict=true by default (opt-out), then eventually maybe even remove the option.

$ cat test.yaml
looseImage: github.com/google/ko/cmd/ko
strictImage: ko://github.com/google/ko/cmd/ko
$ KO_DOCKER_REPO=gcr.io/foo go run ./cmd/ko/ resolve -f test.yaml
...
looseImage: gcr.io/foo/ko-099ba5bcefdead87f92606265fb99ac0@sha256:dbe1a3e11de9e701c9e7a5e61d4f3c2dc93eee2552bb7f6831a23c1147e6899a
strictImage: gcr.io/foo/ko-099ba5bcefdead87f92606265fb99ac0@sha256:dbe1a3e11de9e701c9e7a5e61d4f3c2dc93eee2552bb7f6831a23c1147e6899a

---
$ KO_DOCKER_REPO=gcr.io/foo go run ./cmd/ko/ resolve -f test.yaml -S
...
looseImage: github.com/google/ko/cmd/ko
strictImage: gcr.io/foo/ko-099ba5bcefdead87f92606265fb99ac0@sha256:dbe1a3e11de9e701c9e7a5e61d4f3c2dc93eee2552bb7f6831a23c1147e6899a
---

Sending this out to get feedback on the overall idea; it sounds like Go modules is going to be pretty painful without this as an option at least.

[imjasonh]

$ cat test.yaml 
looseImage: github.com/google/ko/cmd/ko          # unresolved in strict mode
strictImage: ko://github.com/google/ko/cmd/ko    # resolved in any mode
badImage: github.com/blargh/whatever             # unresolved in either mode
badStrictImage: ko://github.com/blargh/whatever  # resolved in loose mode; fails in strict mode

# loose mode
$ KO_DOCKER_REPO=gcr.io/my-project go run ./cmd/ko/ resolve -f test.yaml
badImage: github.com/blargh/whatever
badStrictImage: ko://github.com/blargh/whatever
looseImage: gcr.io/my-project/ko-099ba5bcefdead87f92606265fb99ac0@sha256:174089f40c1c0f9f5103c7858a39827ed50b80c6a7b877cf63714b8029b95fcf
strictImage: gcr.io/my-project/ko-099ba5bcefdead87f92606265fb99ac0@sha256:174089f40c1c0f9f5103c7858a39827ed50b80c6a7b877cf63714b8029b95fcf

# strict mode
$ KO_DOCKER_REPO=gcr.io/my-project go run ./cmd/ko/ resolve -S -f test.yaml
2019/08/15 09:42:47 error processing import paths in "test.yaml": Strict reference "ko://github.com/blargh/whatever" is not supported

If you remove badStrictImage the strict-mode resolved output is:

badImage: github.com/blargh/whatever
looseImage: github.com/google/ko/cmd/ko
strictImage: gcr.io/test-argo/ko-099ba5bcefdead87f92606265fb99ac0@sha256:174089f40c1c0f9f5103c7858a39827ed50b80c6a7b877cf63714b8029b95fcf

Non-ko://-prefixed paths are ignored even if they're otherwise supported. Only paths that begin with ko:// are considered in strict mode.

[jonjohnsonjr]

a couple nits but lgtm

[jonjohnsonjr]

I'm mildly -1 on including -S -- we will eventually run out of flags if we keep this up :P

[imjasonh]

Yeah, I kind of agree. I'll remove -S until someone complains it's too much to type.