Update earthly/dind Docker tag to alpine-3.20-docker-26.1.3-r0 (#70)

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This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| earthly/dind | minor | `alpine-3.19-docker-25.0.5-r1` ->
`alpine-3.20-docker-26.1.3-r0` |

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docs/dockerhub.md

@@ -11,7 +11,7 @@ This image supports 3 Linux distributions:
 * ubuntu:24.04
 
 For which the current latest tags (respectively) are:
-* `alpine-3.19-docker-25.0.5-r1`
+* `alpine-3.20-docker-26.1.3-r0`
 * `ubuntu-20.04-docker-26.1.3-1`
 * `ubuntu-23.04-docker-25.0.2-1`
 * `ubuntu-24.04-docker-26.1.3-1`