feat: Add gitpod configuration copied from mathlib4 (#229)

Notably this ensures a version of ubuntu on which `lake exe cache` works.

In addition to what mathlib4 needs, this installs the relevant C++ compiler tools and the `jq` command line.

.docker/gitpod/Dockerfile

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+# This is the Dockerfile for `leanprovercommunity/mathlib:gitpod`.
+
+# 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 && \
+    # mathport specific
+    apt-get install -y git libgmp-dev cmake ccache clang jq && \
+    # mathlib4
+    apt-get install sudo git curl git bash-completion python3 -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 mathlib is currently using
+RUN . ~/.profile && elan toolchain install $(curl https://raw.githubusercontent.com/leanprover-community/mathlib4/master/lean-toolchain)
+
+ENV PATH="/home/gitpod/.local/bin:/home/gitpod/.elan/bin:${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
+
+vscode:
+  extensions:
+    - leanprover.lean4
+
+tasks:
+  - init: lake exe cache get