fix(build-and-push): stop DOCKER_HOST overriding remote-builder context

- DOCKER_HOST env var was set to the builder hostname, which Docker treats
  as tcp://host:2375 and which overrides the active docker context, so the
  SSH remote-builder context was ignored (Cannot connect to tcp://docker-build.haven:2375)
- context endpoint used a literal ssh://***@ instead of the docker_user input
- create an actual buildx builder instance named remote-builder (build-push-action
  needs a builder instance, not just a context)

build-and-push/action.yaml

@@ -63,22 +63,35 @@ runs:
       shell: bash
       env:
         SSH_KEY: ${{ inputs.ssh_key }}
-        DOCKER_HOST: ${{ inputs.docker_host }}
+        BUILDER_HOST: ${{ inputs.docker_host }}
       run: |
         mkdir -p ~/.ssh
         echo "${SSH_KEY}" > ~/.ssh/id_ed25519
         chmod 600 ~/.ssh/id_ed25519
-        ssh-keyscan "${DOCKER_HOST}" >> ~/.ssh/known_hosts
+        ssh-keyscan "${BUILDER_HOST}" >> ~/.ssh/known_hosts
 
     - name: Set up remote Docker Buildx builder
       shell: bash
       env:
-        DOCKER_HOST: ${{ inputs.docker_host }}
-        DOCKER_USER: ${{ inputs.docker_user }}
+        BUILDER_HOST: ${{ inputs.docker_host }}
+        BUILDER_USER: ${{ inputs.docker_user }}
       run: |
+        # Never set the DOCKER_HOST env var to the builder hostname: a bare
+        # value is treated as tcp://host:2375 and, more importantly, DOCKER_HOST
+        # *overrides* the active docker context — which is exactly what broke
+        # the previous version (it tried tcp://docker-build.haven:2375 instead
+        # of the SSH context). We drive everything through a context instead.
+        unset DOCKER_HOST || true
+
+        docker context rm -f remote-builder 2>/dev/null || true
         docker context create remote-builder \
-          --docker "host=ssh://${DOCKER_USER}@${DOCKER_HOST}"
-        docker context use remote-builder
+          --docker "host=ssh://${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.
+        docker buildx rm -f remote-builder 2>/dev/null || true
+        docker buildx create --name remote-builder --context remote-builder --use
         docker buildx inspect --bootstrap
 
     - name: Generate tags list