docker: Update image to golang:1.22.6-alpine3.20.

This updates the docker image to golang:1.22.6-alpine3.20.

To confirm the new digest:

```
$ docker pull golang:1.22.6-alpine3.20
1.22.6-alpine3.20: Pulling from library/golang
...
Digest: sha256:1a478681b671001b7f029f94b5016aed984a23ad99c707f6a0ab6563860ae2f3
...
```

contrib/docker/Dockerfile

@@ -3,13 +3,13 @@
 ###############
 
 # Basic Go environment with git, SSL CA certs, and upx.
-# The image below is golang:1.22.5-alpine3.20 (linux/amd64)
+# The image below is golang:1.22.6-alpine3.20 (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@sha256:8c9183f715b0b4eca05b8b3dbf59766aaedb41ec07477b132ee2891ac0110a07 AS builder
+FROM golang@sha256:1a478681b671001b7f029f94b5016aed984a23ad99c707f6a0ab6563860ae2f3 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