Revert "Require continuation char to be last char in a line"

This reverts commit 105bc63295,
which (although correct), resulted in a backward incompatible
change.

We can re-implement this in future, after this changes goes
through a deprecation cycle

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/parser/parser.go

@@ -61,7 +61,7 @@ func SetEscapeToken(s string, d *Directive) error {
 		return fmt.Errorf("invalid ESCAPE '%s'. Must be ` or \\", s)
 	}
 	d.EscapeToken = rune(s[0])
-	d.LineContinuationRegex = regexp.MustCompile(`\` + s + `$`)
+	d.LineContinuationRegex = regexp.MustCompile(`\` + s + `[ \t]*$`)
 	return nil
 }
 

builder/dockerfile/parser/testfiles/continueIndent/Dockerfile

@@ -13,6 +13,8 @@ world
 RUN echo hello \
 goodbye\
 frog
+RUN echo hello  \  
+world
 RUN echo hi \
  \
  world \

builder/dockerfile/parser/testfiles/continueIndent/result

@@ -3,6 +3,7 @@
 (run "echo hello    world")
 (run "echo hello  world")
 (run "echo hello goodbyefrog")
+(run "echo hello  world")
 (run "echo hi   world  goodnight")
 (run "echo goodbyefrog")
 (run "echo goodbyefrog")

integration-cli/docker_cli_build_test.go

@@ -7043,17 +7043,6 @@ func (s *DockerSuite) TestBuildCmdShellArgsEscaped(c *check.C) {
 	}
 }
 
-func (s *DockerSuite) TestContinueCharSpace(c *check.C) {
-	// Test to make sure that we don't treat a \ as a continuation
-	// character IF there are spaces (or tabs) after it on the same line
-	name := "testbuildcont"
-	_, err := buildImage(name, "FROM busybox\nRUN echo hi \\\t\nbye", true)
-	c.Assert(err, check.NotNil, check.Commentf("Build 1 should fail - didn't"))
-
-	_, err = buildImage(name, "FROM busybox\nRUN echo hi \\ \nbye", true)
-	c.Assert(err, check.NotNil, check.Commentf("Build 2 should fail - didn't"))
-}
-
 // Test case for #24912.
 func (s *DockerSuite) TestBuildStepsWithProgress(c *check.C) {
 	name := "testbuildstepswithprogress"