fix: bump Docker Go version from 1.22 to 1.25 to match go.mod

go.mod requires go >= 1.25.0 but both Dockerfiles used golang:1.22-alpine,
causing `go mod download` to fail during container build.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -12,7 +12,7 @@ COPY web/ .
 RUN npm run build
 
 # Stage 2: Build Go binary
-FROM golang:1.22-alpine AS builder
+FROM golang:1.25-alpine AS builder
 
 RUN apk add --no-cache git ca-certificates tzdata
 

Dockerfile.agent

@@ -1,6 +1,6 @@
 # Multi-stage build for certctl agent
 # Stage 1: Build
-FROM golang:1.22-alpine AS builder
+FROM golang:1.25-alpine AS builder
 
 RUN apk add --no-cache git ca-certificates