docker: Update image to golang:1.21.1-alpine3.18.

This updates the docker image to golang:1.21.1-alpine3.18.

To confirm the new digest:

```
$ docker pull golang:1.21.1-alpine3.18
1.21.1-alpine3.18: Pulling from library/golang
...
Digest: sha256:96634e55b363cb93d39f78fb18aa64abc7f96d372c176660d7b8b6118939d97b
...
```

contrib/docker/Dockerfile

@@ -3,13 +3,13 @@
 ###############
 
 # Basic Go environment with git, SSL CA certs, and upx.
-# The image below is golang:1.21.0-alpine3.18 (linux/amd64)
+# The image below is golang:1.21.1-alpine3.18 (linux/amd64)
 # It's pulled by the digest (immutable id) to avoid supply-chain attacks.
 # Maintainer Note:
 #    To update to a new digest, you must first manually pull the new image:
 #    `docker pull golang:<new version>`
 #    Docker will print the digest of the new image after the pull has finished.
-FROM golang@sha256:445f34008a77b0b98bf1821bf7ef5e37bb63cc42d22ee7c21cc17041070d134f AS builder
+FROM golang@sha256:96634e55b363cb93d39f78fb18aa64abc7f96d372c176660d7b8b6118939d97b AS builder
 RUN apk add --no-cache git ca-certificates upx
 
 # Empty directory to be copied into place in the production image since it will