Add support for passing secrets as text (#78)

[charles-dyfis-net]

To preserve the existing meaning of BUILDKIT_SECRET_, a new BUILDKIT_SECRETTEXT_ prefix is added, which writes content to a file before adding corresponding entry to the BuildkitSecrets map.

These contents are created in buildkit-secrets/ inside the default temporary directory; since the Docker container this all happens in is transient, no need to clean them up.

See #78 for motivation.

[taylorsilva]

We should add a test for this new env var. Let me see where that would go. I'll drop you some pointers / a test you can copy-paste from.

[taylorsilva]

You can use this test as as starting point:

oci-build-task/task_test.go

Lines 253 to 259 in a46d096

	func (s *TaskSuite) TestBuildkitSecrets() {
	s.req.Config.ContextDir = "testdata/buildkit-secret"
	s.req.Config.BuildkitSecrets = map[string]string{"secret": "testdata/buildkit-secret/secret"}
	_, err := s.build()
	s.NoError(err)
	}

That test makes use of the test data here: https://github.com/concourse/oci-build-task/tree/master/testdata/buildkit-secret
You can probably re-use that Dockerfile. If it doesn't work out just make a new folder with a Dockerfile that works for you.

[charles-dyfis-net]

@taylorsilva -- do the changes meet your expectations wrt test suite coverage?

[taylorsilva]

LGTM - test coverage is fine. Thanks for the PR!