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>

Dockerfile

@@ -1,7 +1,7 @@
 ### BUILDER STAGE
 #
 #
-FROM golang:1.26-alpine AS builder
+FROM golang:1.26-alpine3.23 AS builder
 
 # Do not remove `git` here, it is required for getting runner version when executing `make build`
 RUN apk add --no-cache make git
@@ -17,7 +17,7 @@ RUN make clean && make build
 ### DIND VARIANT
 #
 #
-FROM docker:29-dind AS dind
+FROM docker:29.5.2-dind AS dind
 
 ARG VERSION=dev
 
@@ -37,7 +37,7 @@ ENTRYPOINT ["s6-svscan","/etc/s6"]
 ### DIND-ROOTLESS VARIANT
 #
 #
-FROM docker:29-dind-rootless AS dind-rootless
+FROM docker:29.5.2-dind-rootless AS dind-rootless
 
 ARG VERSION=dev
 
@@ -63,7 +63,7 @@ ENTRYPOINT ["s6-svscan","/etc/s6"]
 ### BASIC VARIANT
 #
 #
-FROM alpine AS basic
+FROM alpine:3.23 AS basic
 
 ARG VERSION=dev
 

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