Make the registry hostname configurable and drop the CA mount

buildDocker.groovy hardcoded harbor.35.238.248.203.nip.io as the push
target, so the registry could not move without editing this shared library
and every consumer moving in the same commit. It now reads
config.harbor_registry, whose default lives in homelabPipeline.groovy
beside harbor_project and every other key.

The stage errors rather than defaulting when the value is missing. Carrying
a second copy of the literal would leave two defaults free to disagree, and
an unset value would otherwise build an image named "null/<project>/<repo>"
— which docker accepts as a hostname and then fails to resolve, pointing
nowhere near the cause.

The dind pod no longer mounts the registry CA. That mount existed because
the registry was a nip.io name, which no public CA will issue for, so
cert-manager signed Harbor from a private CA; the node pool was told to
trust it for pulls, but a push comes from dockerd inside the build pod,
which has its own trust store. harbor.infra.deployshed.com carries a Let's
Encrypt certificate that both already trust, so the mount, its volume and
the whole arrangement go away rather than being repointed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LEsTefWWifp4ikvhHF5s6N

resources/com/homelab/go-Dockerfile

@@ -13,14 +13,14 @@
 # a dockerBuildVersion whose tag isn't in devops-base-images/images.txt
 # yet needs that added and re-mirrored first, unlike pulling straight
 # from Docker Hub where any tag "just worked".
-FROM harbor.35.238.248.203.nip.io/base-images/golang:${version}-alpine AS build
+FROM harbor.infra.deployshed.com/base-images/golang:${version}-alpine AS build
 WORKDIR /src
 COPY go.mod go.sum* ./
 RUN go mod download 2>/dev/null || true
 COPY . .
 RUN CGO_ENABLED=0 go build -o /app .
 
-FROM harbor.35.238.248.203.nip.io/base-images/alpine:3.20
+FROM harbor.infra.deployshed.com/base-images/alpine:3.20
 COPY --from=build /app /app
 EXPOSE 8080
 ENTRYPOINT ["/app"]