Configure BuildKit to allow plain HTTP to the local registry

Docker defaults to HTTPS for any non-docker.io registry, and
10.0.0.246:3000 (a bare LAN IP:port) isn't serving valid HTTPS -- the
push kept going out over HTTPS and failing. Adds a buildkitd config
telling BuildKit specifically to use HTTP for that host.

This only fixes the actual build+push step (BuildKit). The "Log in"
step runs a plain `docker login` through the classic Docker CLI/daemon
instead, which doesn't read this config -- that one still needs
10.0.0.246:3000 added to "insecure-registries" in the actual Docker
daemon's /etc/docker/daemon.json on whatever host runs the Gitea
Actions runner, followed by a daemon restart. That's runner-host
infrastructure outside this repo.

.gitea/workflows/server-docker-build.yml

@@ -16,6 +16,22 @@ jobs:
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
+        with:
+          # 10.0.0.246:3000 is plain HTTP (a bare LAN IP:port, no TLS cert
+          # would validate for it), but BuildKit defaults to HTTPS for any
+          # registry that isn't docker.io. This tells BuildKit specifically
+          # to skip that for this one host. NOTE: this only covers the
+          # actual build+push (BuildKit). The "Log in" step below runs a
+          # plain `docker login`, which goes through the classic Docker
+          # CLI/daemon instead of BuildKit and does NOT read this config --
+          # that one only works once the Docker daemon backing this runner
+          # has 10.0.0.246:3000 listed under "insecure-registries" in its
+          # own /etc/docker/daemon.json (then `systemctl restart docker`).
+          # That's runner-host infrastructure this repo can't configure.
+          config-inline: |
+            [registry."10.0.0.246:3000"]
+              http = true
+              insecure = true
 
       - name: Log in to local Gitea Container Registry
         uses: docker/login-action@v3