some starting point

.gitignore

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+/etc-portage/*
+/portajohn-db/*

README.asc

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+== portajohn
+
+Since John came up with the idea of this portage hack, this project is called 'portajohn' in John's honor.
+
+
+=== run it
+
+  docker-compose build
+
+  docker-compose run portajohn /bin/bash
+

docker-compose.yaml

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+version: "3.6"
+services:
+  portajohn:
+    build:
+      context: ./ubuntu
+      dockerfile: Dockerfile
+    environment:
+      - USER_UID=1000
+      - USER_GID=1000
+#   restart: always
+    volumes:
+      - ./etc-portage:/etc/portage
+      - ./portajohn-db:/var/db/portajohn-db
+      - /etc/timezone:/etc/timezone:ro
+      - /etc/localtime:/etc/localtime:ro
+

ubuntu/Dockerfile

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+FROM ubuntu:20.04
+
+VOLUME /etc/portage
+VOLUME /var/db/portajohn-db
+
+RUN apt-get update && apt-get install -y \
+    git \
+    zstd \
+    python3 \
+    python3-distutils \
+    python3-pip \
+    build-essential \
+    rsync
+
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y tzdata
+
+RUN apt-get install -y mc
+
+RUN useradd -U -r -s /bin/false portage
+
+WORKDIR /portajohn
+
+RUN git clone https://github.com/gentoo/portage.git; \
+    cd portage; \
+    python3 setup.py install; \
+    pip3 install gemato
+
+
+RUN cd /; \
+    emerge --version
+
+CMD ["/bin/bash"]