Fix detection for missing parameter in substitution

`${}`, `${:}` and so on are invalid because there's
no parameter within the brackets; fix detection for
this situation and add/update tests.

There were some existing test-cases that were testing
for the wrong behavior, which are now updated.

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

rewritten from github.com/moby/moby 334bf3ea76004d0abe02dd1698989f9eaf87a86a

frontend/dockerfile/shell/envVarTest

@@ -29,10 +29,14 @@ A|he\$PWD                  |     he$PWD
 A|he\\$PWD                 |     he\/home
 A|"he\$PWD"                |     he$PWD
 A|"he\\$PWD"               |     he\/home
+A|\${}                     |     ${}
+A|\${}aaa                  |     ${}aaa
 A|he\${}                   |     he${}
 A|he\${}xx                 |     he${}xx
-A|he${}                    |     he
-A|he${}xx                  |     hexx
+A|${}                      |     error
+A|${}aaa                   |     error
+A|he${}                    |     error
+A|he${}xx                  |     error
 A|he${hi}                  |     he
 A|he${hi}xx                |     hexx
 A|he${PWD}                 |     he/home
@@ -88,8 +92,8 @@ A|안녕\$PWD                  |     안녕$PWD
 A|안녕\\$PWD                 |     안녕\/home
 A|안녕\${}                   |     안녕${}
 A|안녕\${}xx                 |     안녕${}xx
-A|안녕${}                    |     안녕
-A|안녕${}xx                  |     안녕xx
+A|안녕${}                    |     error
+A|안녕${}xx                  |     error
 A|안녕${hi}                  |     안녕
 A|안녕${hi}xx                |     안녕xx
 A|안녕${PWD}                 |     안녕/home
@@ -129,4 +133,7 @@ A|${aaa:-bbb}               |     bbb
 A|${aaa:-${bbb:-ccc}}       |     ccc
 A|${aaa:-bbb ${foo}         |     error
 A|${aaa:-bbb {foo}          |     bbb {foo
+A|${:}                      |     error
+A|${:-bbb}                  |     error
+A|${:+bbb}                  |     error
 

frontend/dockerfile/shell/lex.go

@@ -261,23 +261,22 @@ func (sw *shellWord) processDollar() (string, error) {
 	}
 
 	sw.scanner.Next()
+	switch sw.scanner.Peek() {
+	case scanner.EOF:
+		return "", errors.New("syntax error: missing '}'")
+	case '{', '}', ':':
+		// Invalid ${{xx}, ${:xx}, ${:}. ${} case
+		return "", errors.New("syntax error: bad substitution")
+	}
 	name := sw.processName()
-	ch := sw.scanner.Peek()
+	ch := sw.scanner.Next()
 	switch ch {
 	case '}':
 		// Normal ${xx} case
-		sw.scanner.Next()
 		return sw.getEnv(name), nil
-	case '{':
-		// Invalid ${{xx} case
-		return "", errors.New("syntax error: bad substitution")
-	case scanner.EOF:
-		return "", errors.New("syntax error: missing '}'")
 	case ':':
 		// Special ${xx:...} format processing
 		// Yes it allows for recursive $'s in the ... spot
-
-		sw.scanner.Next() // skip over :
 		modifier := sw.scanner.Next()
 
 		word, _, err := sw.processStopOn('}')