Dockerfile: update xx to v1.4.0

[thaJeztah]

full diff: tonistiigi/xx@v1.2.1...v1.4.0

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Hm... looks like this test is flaky;

=== Failed
=== FAIL: amd64.integration.plugin.logging TestReadPluginNoRead/explicitly_enabled_caching (0.88s)
    read_test.go:90: assertion failed: strings.TrimSpace(buf.String()) is not "hello world": []
    --- FAIL: TestReadPluginNoRead/explicitly_enabled_caching (0.88s)

=== FAIL: amd64.integration.plugin.logging TestReadPluginNoRead (4.83s)
    read_test.go:93: [db86db6a18064] daemon is not started

[thaJeztah]

• opened Flaky test: TestReadPluginNoRead #47573 to track the flaky test

[thaJeztah]

All green now 🎉 @crazy-max ptal 🤗