docker: bump golang from 1.20.7-alpine3.18 to 1.21.0-alpine3.18

Bumps golang from 1.20.7-alpine3.18 to 1.21.0-alpine3.18.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20.7-alpine3.18@sha256:b255d9352ff4967f0424731acf5a262c3bb4b5bd712e6324ad9ed533c9d2aee7 AS build
+FROM golang:1.21.0-alpine3.18@sha256:445f34008a77b0b98bf1821bf7ef5e37bb63cc42d22ee7c21cc17041070d134f AS build
 
 COPY . /go/src/github.com/jauderho/bl3auto
 WORKDIR /go/src/github.com/jauderho/bl3auto