fix(dockerfile): Support FROM --arg=$value

[anguslees]

Changes:

In multi-platform Dockerfiles using docker buildx, it is common to
use build arg substition such as:

FROM --platform=$BUILDPLATFORM image:tag

This change extends the renovate regex to include $ (actually all
non-whitespace characters) in FROM parameters.

Closes #12122

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Fyi, the modified testcase without the patch reproduces the original issue and fails with the following (because it doesn't see the FROM --platform=$arg line at all):

test output

  ● manager/dockerfile/extract › extractPackageFile() › extracts images from all sorts of (maybe multiline) FROM and COPY --from statements

    expect(received).toMatchSnapshot()

    Snapshot name: `manager/dockerfile/extract extractPackageFile() extracts images from all sorts of (maybe multiline) FROM and COPY --from statements 1`

    - Snapshot  - 4
    + Received  + 2

    @@ -17,16 +17,14 @@
          "depType": "stage",
          "replaceString": "image2:1.0.0@sha256:abcdef",
        },
        Object {
          "autoReplaceStringTemplate": "{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
    -     "currentDigest": undefined,
    -     "currentValue": undefined,
          "datasource": "docker",
    -     "depName": "image4",
          "depType": "stage",
    -     "replaceString": "image4",
    +     "replaceString": "--platform=$arg",
    +     "skipReason": "contains-variable",
        },
        Object {
          "autoReplaceStringTemplate": "{{depName}}{{#if newValue}}:{{newValue}}{{/if}}{{#if newDigest}}@{{newDigest}}{{/if}}",
          "currentDigest": undefined,
          "currentValue": undefined,

      243 |     it('extracts images from all sorts of (maybe multiline) FROM and COPY --from statements', () => {
      244 |       const res = extractPackageFile(d2).deps;
    > 245 |       expect(res).toMatchSnapshot();
          |                   ^
      246 |       expect(res).toHaveLength(9);
      247 |     });
      248 |     it('handles calico/node', () => {

      at Object.<anonymous> (lib/manager/dockerfile/extract.spec.ts:245:19)

 › 1 snapshot failed.

[renovate-release]

🎉 This PR is included in version 28.3.0 🎉

The release is available on:

• GitHub release
• 28.3.0

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