Add org.opencontainers.image.source to Dockerfiles (#9211)

### Purpose

The OCI image spec specifies well-defined
[annotations](https://github.com/opencontainers/image-spec/blob/main/annotations.md)
that can be added to images.
Theses annotations can then be used by other tools to gather more
information of an image.

This PR adds the `org.opencontainers.image.source` to allow tools such
as [renovate](https://github.com/renovatebot/renovate) to find the
release notes of a give version.

~~I've only done this change for `Dockerfile`. Should I also add the
label to the other dockerfiles?~~
I've now added the source annotations to all `Dockerfile`s & action
workflows.

### Testing

None, change was done by following the [renovate
documentation](https://docs.renovatebot.com/modules/datasource/docker/).

Dockerfile.builder

@@ -1,6 +1,14 @@
 ARG GOVERSION=latest
 FROM golang:$GOVERSION
 
+LABEL org.opencontainers.image.authors="The Syncthing Project" \
+      org.opencontainers.image.url="https://syncthing.net" \
+      org.opencontainers.image.documentation="https://docs.syncthing.net" \
+      org.opencontainers.image.source="https://github.com/syncthing/syncthing" \
+      org.opencontainers.image.vendor="The Syncthing Project"
+      org.opencontainers.image.licenses="MPL-2.0"
+      org.opencontainers.image.title="Syncthing Builder"
+
 # FPM to build Debian packages
 RUN apt-get update && apt-get install -y --no-install-recommends \
 	locales rubygems ruby-dev build-essential git \