[dockerfile] Install pip and properly configure pixi

Dockerfile

 FROM ghcr.io/prefix-dev/pixi:latest AS build
 
+COPY config.toml /root/.config/pixi/config.toml
 COPY pyproject.toml pixi.lock /app/
 WORKDIR /app
 RUN pixi install --frozen -e cuda-base

pyproject.toml

@@ -174,6 +174,7 @@ cuda = "12.1"
 [tool.pixi.feature.cuda.target.linux-64.dependencies]
 #cuda = { version = "*", channel = "nvidia" }
 pytorch-cuda = { version = "12.1.*", channel = "pytorch" }
+pip = "*"  # required for docker image building
 
 [tool.pixi.environments]
 default = { features = ["qa", "build", "doc", "test", "dev", "py312", "self"] }