fix(build-and-push): create buildx builder from SSH endpoint, drop colliding context

buildx refuses a builder instance whose name matches an existing docker
context. Create the builder directly from the ssh:// endpoint and clean up
any leftover context/builder of the same name first (idempotent re-runs).

build-and-push/action.yaml

@@ -78,20 +78,20 @@ runs:
       run: |
         # Never set the DOCKER_HOST env var to the builder hostname: a bare
         # value is treated as tcp://host:2375 and, more importantly, DOCKER_HOST
-        # *overrides* the active docker context — which is exactly what broke
-        # the previous version (it tried tcp://docker-build.haven:2375 instead
-        # of the SSH context). We drive everything through a context instead.
+        # *overrides* the active docker context/builder. Keep it unset so buildx
+        # talks to the SSH endpoint we specify below.
         unset DOCKER_HOST || true
 
-        docker context rm -f remote-builder 2>/dev/null || true
-        docker context create remote-builder \
-          --docker "host=ssh://${BUILDER_USER}@${BUILDER_HOST}"
-
-        # build-push-action's `builder: remote-builder` needs a *buildx builder
-        # instance* named remote-builder, not just a context. Create it from the
-        # SSH context so it resolves to the remote engine over SSH.
+        # Create the buildx builder directly from the SSH endpoint. We do NOT
+        # create a docker context of the same name first: buildx refuses to
+        # create a builder instance whose name collides with an existing
+        # context ("instance name already exists as context builder").
         docker buildx rm -f remote-builder 2>/dev/null || true
-        docker buildx create --name remote-builder --use remote-builder
+        docker context rm -f remote-builder 2>/dev/null || true
+        docker buildx create \
+          --name remote-builder \
+          --use \
+          "ssh://${BUILDER_USER}@${BUILDER_HOST}"
         docker buildx inspect --bootstrap
 
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