[api/client] Fix build when context dir is symlink

[jlhawn]

Symbolic links in the context directory path are now evaluated.

fixes #15037

[crosbymichael]

@jlhawn does this need to go into 1.8?

[icecrime]

@crosbymichael I guess it does: looks like a regression from the docker cp notary PR.

[jlhawn]

I've added a second commit with a test simple test case. With the test case commit alone the test case fails with the same error that @crosbymichael saw in #15037 but with the api/client/build.go patch commit it passes.

[calavera]

@jlhawn can you squash them together when it's ready? it will be easier to track/cherry-pick the changes.

[jlhawn]

@calavera done.

[jlhawn]

hmm, there are still some CI errors, but they seem unrelated.

[calavera]

LGTM.

[crosbymichael]

LGTM

[jlhawn]

hmm, this test still failed on Windows:

FAIL: docker_cli_build_test.go:4745: DockerSuite.TestBuildRenamedDockerfile

docker_cli_build_test.go:4771:
    c.Fatal(err)
... Error: "build -f files\\Dockerfile -t test2 ." failed with errors: exit status 1 : "unable to prepare context: The Dockerfile (C:\\Windows\\TEMP\\fake-context137025938\\files\\Dockerfile) must be within the build context (.)\n")

[calavera]

@jlhawn I'm not sure symlinks work on windows. We might need to extract that into platform specific files.

/cc @jhowardmsft

[lowenna]

Yes, I believe that will be needed.

[jlhawn]

@calavera @jhowardmsft I've opened #15060 to try and fix that test case on Windows. There's no need to use symlink.FollowSymlinkInScope here anyway.