fix: upgrade Go builder image to 1.25-bookworm

[Copilot]

gin-gonic/gin@v1.12.0 (resolved at build time via go mod tidy) requires Go >= 1.25.0, causing the automated Docker build to fail on the existing golang:1.24-bullseye image.

Change

# Before
FROM golang:1.24-bullseye AS backend-builder

# After
FROM golang:1.25-bookworm AS backend-builder

Upgrades the backend-builder stage base image to golang:1.25-bookworm, satisfying gin v1.12.0's minimum toolchain requirement without pinning any dependency versions.

Original prompt

Problem

The weekly automated Docker build is failing because gin-gonic/gin@v1.12.0 requires Go >= 1.25.0, but the Dockerfile uses golang:1.24-bullseye (Go 1.24.6).

Error:

go: toolchain upgrade needed to resolve github.com/gin-gonic/gin
go: github.com/gin-gonic/gin@v1.12.0 requires go >= 1.25.0 (running go 1.24.6; GOTOOLCHAIN=local)

Since there is no committed go.mod, go mod tidy at build time resolves the latest gin — now v1.12.0 — which Go 1.24 cannot satisfy.

Fix Required

In Dockerfile, STAGE 2 (backend-builder), change the base image from golang:1.24-bullseye to golang:1.25-bookworm.

Change:

FROM golang:1.24-bullseye AS backend-builder

To:

FROM golang:1.25-bookworm AS backend-builder

This is the only change needed. Do NOT add any go get pin lines. Do NOT modify any other part of the Dockerfile.

Note: there is already an open PR (#17) that takes a different approach (pinning gin to v1.10.0). This new PR takes the preferred approach of upgrading the Go image instead, so gin v1.12.0 is fully supported. Please open a separate, new PR for this change.

This pull request was created from Copilot chat.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.