feat(manager/dockerfile): Add syntax statement support

lib/modules/manager/dockerfile/extract.spec.ts

@@ -947,6 +947,16 @@ describe('modules/manager/dockerfile/extract', () => {
     it('handles an alternative escape character', () => {
       const res = extractPackageFile(d4, '', {})?.deps;
       expect(res).toEqual([
+        {
+          autoReplaceStringTemplate:
+            '{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}',
+          currentDigest: undefined,
+          currentValue: '1',
+          datasource: 'docker',
+          depName: 'docker/dockerfile',
+          depType: 'syntax',
+          replaceString: 'docker/dockerfile:1',
+        },
         {
           autoReplaceStringTemplate:
             ' ARG `\n' +
@@ -1158,6 +1168,38 @@ describe('modules/manager/dockerfile/extract', () => {
     });
   });
 
+  it('handles # syntax statements', () => {
+    const res = extractPackageFile(
+      '# syntax=docker/dockerfile:1.1.7\n' + 'FROM alpine:3.13.5\n',
+      '',
+      {}
+    );
+    expect(res).toEqual({
+      deps: [
+        {
+          autoReplaceStringTemplate:
+            '{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}',
+          currentDigest: undefined,
+          currentValue: '1.1.7',
+          datasource: 'docker',
+          depName: 'docker/dockerfile',
+          depType: 'syntax',
+          replaceString: 'docker/dockerfile:1.1.7',
+        },
+        {
+          autoReplaceStringTemplate:
+            '{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}',
+          currentDigest: undefined,
+          currentValue: '3.13.5',
+          datasource: 'docker',
+          depName: 'alpine',
+          depType: 'final',
+          replaceString: 'alpine:3.13.5',
+        },
+      ],
+    });
+  });
+
   describe('getDep()', () => {
     it('rejects null', () => {
       expect(getDep(null)).toEqual({ skipReason: 'invalid-value' });

lib/modules/manager/dockerfile/extract.ts

@@ -283,6 +283,25 @@ export function extractPackageFile(
       instruction += '\n' + lineLookahead;
     }
 
+    const syntaxRegex = regEx('^#[ \\t]*syntax=(?<image>\\S+)', 'im');
+    const syntaxMatch = instruction.match(syntaxRegex);
+    if (syntaxMatch?.groups?.image) {
+      const syntaxImage = syntaxMatch.groups.image;
+      const lineNumberRanges: number[][] = [[lineNumberInstrStart, lineNumber]];
+      const dep = getDep(syntaxImage, true, config.registryAliases);
+      dep.depType = 'syntax';
+      processDepForAutoReplace(dep, lineNumberRanges, lines, lineFeed);
+      logger.trace(
+        {
+          depName: dep.depName,
+          currentValue: dep.currentValue,
+          currentDigest: dep.currentDigest,
+        },
+        'Dockerfile # syntax'
+      );
+      deps.push(dep);
+    }
+
     const argRegex = regEx(
       '^[ \\t]*ARG(?:' +
         escapeChar +
@@ -399,7 +418,10 @@ export function extractPackageFile(
   if (!deps.length) {
     return null;
   }
-  for (const d of deps) {
+  if (deps[0].depType !== 'syntax') {
+    deps[0].depType = 'stage';
+  }
+  for (const d of deps.slice(1)) {
     d.depType = 'stage';
   }
   deps[deps.length - 1].depType = 'final';