fix(gomod): use latest go version when binarySource=docker (#9765)

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

lib/manager/gomod/__snapshots__/extract.spec.ts.snap

@@ -3,7 +3,7 @@
 exports[`manager/gomod/extract extractPackageFile() extracts constraints 1`] = `
 Object {
   "constraints": Object {
-    "go": "1.13",
+    "go": "^1.13",
   },
   "deps": Array [
     Object {

lib/manager/gomod/extract.spec.ts

@@ -20,7 +20,7 @@ describe(getName(), () => {
     it('extracts constraints', () => {
       const res = extractPackageFile(gomod3);
       expect(res).toMatchSnapshot();
-      expect(res.constraints.go).toEqual('1.13');
+      expect(res.constraints.go).toEqual('^1.13');
     });
     it('extracts multi-line requires', () => {
       const res = extractPackageFile(gomod2).deps;

lib/manager/gomod/extract.ts

@@ -43,7 +43,7 @@ export function extractPackageFile(content: string): PackageFile | null {
     for (let lineNumber = 0; lineNumber < lines.length; lineNumber += 1) {
       let line = lines[lineNumber];
       if (line.startsWith('go ') && validRange(line.replace('go ', ''))) {
-        constraints.go = line.replace('go ', '');
+        constraints.go = line.replace('go ', '^');
       }
       const replaceMatch = /^replace\s+[^\s]+[\s]+[=][>]\s+([^\s]+)\s+([^\s]+)/.exec(
         line

lib/manager/gomod/readme.md

@@ -3,3 +3,6 @@ You might be interested in the following `postUpdateOptions`:
 1. `gomodTidy` - if you'd like Renovate to run `go mod tidy` after every update before raising the PR.
    1. This is implicitly enabled for major updates
 1. `gomodUpdateImportPaths` - if you'd like Renovate to update your source import paths on major updates before raising the PR.
+
+When Renovate is running using `binarySource=docker` (such as in the hosted WhiteSource Renovate app) then it will pick the latest compatible version of Go to run, i.e. the latest `1.x` release.
+Therefore even if the `go.mod` contains a version like `go 1.14`, you will see Renovate treating that as a `^1.14` constraint and not `=1.14`.