Fix Dockerfile Go version mismatch

[63n0]

This pull request fixes the Docker build failure shown below.

The builder stage uses Go 1.21, while go.mod requires Go 1.24 or higher.
This mismatch causes go mod download to fail during the build.

This PR updates the builder image to Go 1.24 to match the go.mod requirement.

$ sudo docker build .
[+] Building 1.9s (13/17)                                                                  docker:default
 => [internal] load build definition from Dockerfile                                                 0.0s
 => => transferring dockerfile: 384B                                                                 0.0s
 => [internal] load metadata for docker.io/library/alpine:latest                                     1.6s
 => [internal] load metadata for docker.io/library/golang:1.21-alpine3.20                            1.5s
 => [internal] load .dockerignore                                                                    0.0s
 => => transferring context: 2B                                                                      0.0s
 => [builder  1/10] FROM docker.io/library/golang:1.21-alpine3.20@sha256:2414035b086e3c42b99654c8b2  0.0s
 => [internal] load build context                                                                    0.0s
 => => transferring context: 10.35kB                                                                 0.0s
 => [stage-1 1/3] FROM docker.io/library/alpine:latest@sha256:4b7ce07002c69e8f3d704a9c5d6fd3053be50  0.0s
 => CACHED [builder  2/10] RUN apk update                                                            0.0s
 => CACHED [builder  3/10] RUN apk add git                                                           0.0s
 => CACHED [builder  4/10] WORKDIR /app                                                              0.0s
 => [builder  5/10] COPY go.mod ./                                                                   0.0s
 => [builder  6/10] COPY go.sum ./                                                                   0.0s
 => ERROR [builder  7/10] RUN go mod download                                                        0.1s
------                                                                                                    
 > [builder  7/10] RUN go mod download:
0.135 go: go.mod requires go >= 1.24 (running go 1.21.13; GOTOOLCHAIN=local)
------
Dockerfile:10
--------------------
   8 |     COPY go.mod ./
   9 |     COPY go.sum ./
  10 | >>> RUN go mod download
  11 |     
  12 |     COPY . ./
--------------------
ERROR: failed to build: failed to solve: process "/bin/sh -c go mod download" did not complete successfully: exit code: 1

[wind-c]

Thanks! @63n0