Add gitpod configuration

.docker/gitpod/Dockerfile

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+# This file is mostly copied from [mathlib4](https://github.com/leanprover-community/mathlib4/blob/master/.docker/gitpod/Dockerfile)
+
+# gitpod doesn't support multiple FROM statements, (or rather, you can't copy from one to another)
+# so we just install everything in one go
+FROM ubuntu:jammy
+
+USER root
+
+RUN apt-get update && apt-get install sudo git curl bash-completion python3-requests gcc make -y && apt-get clean
+
+RUN useradd -l -u 33333 -G sudo -md /home/gitpod -s /bin/bash -p gitpod gitpod \
+    # passwordless sudo for users in the 'sudo' group
+    && sed -i.bkp -e 's/%sudo\s\+ALL=(ALL\(:ALL\)\?)\s\+ALL/%sudo ALL=NOPASSWD:ALL/g' /etc/sudoers
+USER gitpod
+WORKDIR /home/gitpod
+
+SHELL ["/bin/bash", "-c"]
+
+# gitpod bash prompt
+RUN { echo && echo "PS1='\[\033[01;32m\]\u\[\033[00m\] \[\033[01;34m\]\w\[\033[00m\]\$(__git_ps1 \" (%s)\") $ '" ; } >> .bashrc
+
+# install elan
+RUN curl https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh -sSf | sh -s -- -y --default-toolchain none
+
+# install whichever toolchain batteries is currently using
+RUN . ~/.profile && elan toolchain install $(curl https://raw.githubusercontent.com/leanprover-community/batteries/main/lean-toolchain)
+
+# install neovim (for any lean.nvim user), via tarball since the appimage doesn't work for some reason, and jammy's version is ancient
+RUN curl -s -L https://github.com/neovim/neovim/releases/download/stable/nvim-linux64.tar.gz | tar xzf - && sudo mv nvim-linux64 /opt/nvim
+
+ENV PATH="/home/gitpod/.local/bin:/home/gitpod/.elan/bin:/opt/nvim/bin:${PATH}"
+
+# fix the infoview when the container is used on gitpod:
+ENV VSCODE_API_VERSION="1.50.0"
+
+# ssh to github once to bypass the unknown fingerprint warning
+RUN ssh -o StrictHostKeyChecking=no github.com || true
+
+# run sudo once to suppress usage info
+RUN sudo echo finished

.gitpod.yml

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+image:
+  file: .docker/gitpod/Dockerfile
+
+additionalRepositories:
+ - url: https://github.com/leanprover-community/mathlib4
+
+vscode:
+  extensions:
+    - leanprover.lean4
+
+tasks:
+  - init: |
+      elan self update
+      cd ../mathlib4
+      git checkout v4.18.0
+      lake exe cache get