Add Dev Container configuration

.devcontainer/Dockerfile

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+FROM mcr.microsoft.com/devcontainers/ruby:2.7-bullseye
+
+# Configure editors and pagers
+ENV EDITOR="nano" \
+    VISUAL="code --wait" \
+    PAGER="less" \
+    LESS="-FMR"
+
+# Silence Ruby deprecation warnings
+ENV RUBYOPT="-W:no-deprecated"
+
+# Configure ruby/debug gem
+ENV RUBY_DEBUG_HISTORY_FILE="/workspace/.rdbg_history" \
+    RUBY_DEBUG_SAVE_HISTORY="1000"
+
+# Configure RubyGems to skip gem documentation
+RUN echo "gem: --no-document" >> ~/.gemrc
+
+WORKDIR /workspace

.devcontainer/devcontainer.json

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+{
+  "name": "webmention-client-ruby",
+  "features": {},
+  "customizations": {},
+  "workspaceFolder": "/workspace",
+  "dockerComposeFile": "docker-compose.yml",
+  "service": "default",
+  "postCreateCommand": "bundle install"
+}

.devcontainer/docker-compose.yml

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+services:
+  default:
+    build:
+      context: ..
+      dockerfile: .devcontainer/Dockerfile
+    command: sleep infinity
+    init: true
+    volumes:
+      - ..:/workspace:cached