chore: pin Docker base images to explicit versions (#992)

Pin floating image tags:

- `golang` → `1.26-alpine3.23`
- `docker` dind variants → `29.5.2`
- `alpine` (basic stage + test fixture) → `3.23`

`ubuntu:24.04` and `scratch` left unchanged (no more-specific tag).

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/runner/pulls/992
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>

act/runner/testdata/actions-environment-and-context-tests/docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3
+FROM alpine:3.23
 
 COPY entrypoint.sh /entrypoint.sh