Add GitHub Codespaces integration

.devcontainer/Dockerfile

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+FROM ubuntu:focal
+ARG DEBIAN_FRONTEND=noninteractive
+ARG VSCODE_USERNAME=vscode
+ARG VSCODE_UID=1000
+ARG VSCODE_GID=1000
+RUN apt-get update \
+    && apt-get install --no-install-recommends -y apt-utils dialog sudo 2>&1 \
+    && groupadd --gid $VSCODE_GID $VSCODE_USERNAME \
+    && useradd -s /bin/bash --uid $VSCODE_UID --gid $VSCODE_GID -m $VSCODE_USERNAME \
+    && echo $VSCODE_USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$VSCODE_USERNAME\
+    && chmod 0440 /etc/sudoers.d/$VSCODE_USERNAME \
+    && apt-get autoremove -y \
+    && apt-get clean -y \
+    && rm -rf /var/lib/apt/lists/*
+RUN apt-get update \
+    && apt-get install --no-install-recommends -y git procps lsb-release \
+           build-essential libtool autotools-dev automake pkg-config \
+           bsdmainutils python3 libevent-dev libboost-system-dev \
+           libboost-filesystem-dev libboost-test-dev libboost-thread-dev \
+           libminiupnpc-dev libzmq3-dev libqt5gui5 libqt5core5a libqt5dbus5 \
+           qttools5-dev qttools5-dev-tools libqrencode-dev libdb-dev \
+           libdb++-dev gdb man manpages manpages-dev manpages-posix lynx links \
+           emacs vim mc cppcheck python3-pip pylint black pyflakes3 \
+           python3-jedi python3-autopep8 black yapf3 isort pylint flake8 mypy \
+           pydocstyle pylama wget curl lsof htop net-tools clang bear \
+    && pip3 install prospector==1.3.0 rope==0.18.0

.devcontainer/devcontainer.json

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+{
+   "postCreateCommand": "./autogen.sh && ./configure --with-incompatible-bdb && make -C src/ config/bitcoin-config.h bench/data/block413567.raw.h test/data/asmap.raw.h",
+   "build": {
+      "dockerfile": "Dockerfile"
+   },
+   "runArgs": [
+      "--cap-add=SYS_PTRACE",
+      "--security-opt",
+      "seccomp=unconfined"
+   ],
+   "remoteUser": "vscode",
+   "settings": {
+      "terminal.integrated.shell.linux": "/bin/bash",
+      "terminal.integrated.shellArgs.linux": ["-l"]
+   },
+   "name": "",
+   "extensions": [
+      "ms-python.python",
+      "ms-vscode.cpptools",
+      "jbenden.c-cpp-flylint"
+   ]
+}

.vscode/c_cpp_properties.json

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+{
+   "configurations": [
+      {
+         "defines": [
+            "HAVE_CONFIG_H"
+         ],
+         "includePath": [
+            "${workspaceFolder}/src/**",
+            "/usr/include/"
+         ],
+         "intelliSenseMode": "gcc-x64",
+         "name": "",
+         "compilerPath": "/usr/bin/gcc",
+         "cppStandard": "gnu++11",
+         "cStandard": "gnu11"
+      }
+   ],
+   "version": 4
+}

.vscode/settings.json

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+{
+   "c-cpp-flylint.clang.enable": false,
+   "c-cpp-flylint.flexelint.enable": false,
+   "extensions.ignoreRecommendations": true,
+   "extensions.showRecommendationsOnlyOnDemand": true,
+   "python.showStartPage": false,
+   "telemetry.enableTelemetry": false
+}