build: bump Go builder images to 1.26.5, set GOTOOLCHAIN=auto

Currency bump: golang builder images -> 1.26.5 (latest stable).

Adds ENV GOTOOLCHAIN=auto to each Go builder stage. The official golang
images set GOTOOLCHAIN=local, so a builder image older than the module's
go directive is a hard mid-build failure. The pin stays the hermetic fast
path; auto is the safety net for the next go.mod bump.

No go.mod go directive was changed. Builder images move; directives stay.

Co-authored-by: Hanzo Dev <[email protected]>

cmd/bridge/Dockerfile

@@ -20,7 +20,8 @@ RUN pnpm -C app/bridge build && \
     cp -r app/bridge/dist /ui-out
 
 # ── 2. Go build ───────────────────────────────────────────────────────────
-FROM golang:1.26.4-alpine AS go-build
+FROM golang:1.26.5-alpine AS go-build
+ENV GOTOOLCHAIN=auto
 WORKDIR /src
 RUN apk add --no-cache git ca-certificates tzdata \
     && echo 'nonroot:x:65532:65532:nonroot:/home/nonroot:/sbin/nologin' >> /etc/passwd \

opnet-watcher/Dockerfile

@@ -1,4 +1,5 @@
-FROM golang:1.26.4-alpine AS build
+FROM golang:1.26.5-alpine AS build
+ENV GOTOOLCHAIN=auto
 WORKDIR /app
 COPY go.mod go.sum ./
 RUN go mod download