fix(docker): bump build-stage Go to 1.25 (matches go.mod)

When v2.0.0 added the golang.org/x/image/webp dependency (used by the
media scrubber for WebP decoding), go mod tidy bumped the module's
minimum Go version in go.mod:

  module go-blockchain
  go 1.25.0

The three Dockerfiles in the repo were still pinned to older images:

  /Dockerfile              FROM golang:1.24-alpine
  /deploy/prod/Dockerfile.slim  FROM golang:1.24-alpine
  /docker/media-sidecar/Dockerfile  FROM golang:1.22-alpine

Result: `docker build` on any of them fails at `go mod download` with
  go: go.mod requires go >= 1.25.0 (running go 1.24.13; GOTOOLCHAIN=local)
because Alpine's golang image pins GOTOOLCHAIN=local to keep the
toolchain reproducible.

Fix: bump all three to golang:1.25-alpine. The media-sidecar module
doesn't actually need 1.25 (it's self-contained and only uses stdlib),
but keeping all three in sync avoids surprise the next time somebody
adds a dep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deploy/prod/Dockerfile.slim

@@ -13,7 +13,7 @@
 # testnet validator.
 
 # ---- build stage ----
-FROM golang:1.24-alpine AS builder
+FROM golang:1.25-alpine AS builder
 WORKDIR /app
 
 COPY go.mod go.sum ./