fix(docker): use golang:1.22-alpine as base builder image

Fixes the docker build error on main, which is due to go1.20 not
understanding the `toolchain` directive in the `go.work` file. This
directive was added automatically by `go work` as part of the tidying
process.

```
 #30 [linux/amd64 builder 13/13] RUN go build   -ldflags "-X github.com/peterldowns/pgmigrate/cmd/pgmigrate/shared.Version=v0.0.7 -X github.com/peterldowns/pgmigrate/cmd/pgmigrate/shared.Commit=37e45fa"   -o /dist/pgmigrate   ./cmd/pgmigrate
```

https://github.com/peterldowns/pgmigrate/actions/runs/9586836776/job/26435591975

Dockerfile

@@ -1,7 +1,7 @@
 # Build Stage
 # Pin to the latest minor version without specifying a patch version so that
 # we always deploy security fixes as soon as they are available.
-FROM golang:1.20-alpine as builder
+FROM golang:1.22-alpine as builder
 RUN apk add build-base git
 
 # Have to put our source in the right place for it to build