internal/gaby: use go1.26.0 in the container image

The go.mod of the project requires go1.26.0, so update
the container version as well. Fix broken cloud build.

Change-Id: I26d95fdce3803a4748c90207b23f4ba5c03fa7e3
Reviewed-on: https://go-review.googlesource.com/c/oscar/+/750121
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/gaby/Dockerfile

diff --git a/internal/gaby/Dockerfile b/internal/gaby/Dockerfile
index 818fa3b..bf0b073 100644
--- a/internal/gaby/Dockerfile
+++ b/internal/gaby/Dockerfile
@@ -20,7 +20,7 @@
 ################################################################
 # Note that the Go version on the next line appears again halfway down this file
 # and in cloudbuild.yaml.
-FROM golang:1.25.0 AS builder
+FROM golang:1.26.0 AS builder
 
 # Set the working directory outside $GOPATH to ensure module mode is enabled.
 WORKDIR /src
@@ -45,7 +45,7 @@
 ################################################################
 # Use a a fresh instance but with less things.
 # Among other benefits, the space savings means more room for /tmp on Cloud Run.
-FROM golang:1.25.0
+FROM golang:1.26.0
 
 LABEL maintainer="Go Oscar Team <oscar-team@google.com>"