build: support more variations on context and dockerfile + iidfile

Signed-off-by: Tibor Vass <tibor@docker.com>

vendor/github.com/docker/docker/pkg/urlutil/urlutil.go

@@ -0,0 +1,52 @@
+// Package urlutil provides helper function to check urls kind.
+// It supports http urls, git urls and transport url (tcp://, …)
+package urlutil // import "github.com/docker/docker/pkg/urlutil"
+
+import (
+	"regexp"
+	"strings"
+)
+
+var (
+	validPrefixes = map[string][]string{
+		"url": {"http://", "https://"},
+
+		// The github.com/ prefix is a special case used to treat context-paths
+		// starting with `github.com` as a git URL if the given path does not
+		// exist locally. The "github.com/" prefix is kept for backward compatibility,
+		// and is a legacy feature.
+		//
+		// Going forward, no additional prefixes should be added, and users should
+		// be encouraged to use explicit URLs (https://github.com/user/repo.git) instead.
+		"git":       {"git://", "github.com/", "git@"},
+		"transport": {"tcp://", "tcp+tls://", "udp://", "unix://", "unixgram://"},
+	}
+	urlPathWithFragmentSuffix = regexp.MustCompile(".git(?:#.+)?$")
+)
+
+// IsURL returns true if the provided str is an HTTP(S) URL.
+func IsURL(str string) bool {
+	return checkURL(str, "url")
+}
+
+// IsGitURL returns true if the provided str is a git repository URL.
+func IsGitURL(str string) bool {
+	if IsURL(str) && urlPathWithFragmentSuffix.MatchString(str) {
+		return true
+	}
+	return checkURL(str, "git")
+}
+
+// IsTransportURL returns true if the provided str is a transport (tcp, tcp+tls, udp, unix) URL.
+func IsTransportURL(str string) bool {
+	return checkURL(str, "transport")
+}
+
+func checkURL(str, kind string) bool {
+	for _, prefix := range validPrefixes[kind] {
+		if strings.HasPrefix(str, prefix) {
+			return true
+		}
+	}
+	return false
+}