feat: Add VS Code DevContainer for simplified development setup

.devcontainer/Dockerfile

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+FROM php:8.4-cli-bookworm
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    # Build tools required for PHP extension configuration
+    autoconf \
+    # PHP extension dependencies
+    libfreetype6-dev \
+    libicu-dev \
+    libjpeg62-turbo-dev \
+    libldap2-dev \
+    libpng-dev \
+    libpq-dev \
+    libsqlite3-dev \
+    libzip-dev \
+    libonig-dev \
+    libldap-common \
+    libenchant-2-dev \
+    default-libmysqlclient-dev \
+    # ImageMagick for Imagick extension
+    libmagickwand-dev \
+    # Tools
+    nodejs \
+    npm \
+    aspell \
+    aspell-en \
+    hunspell-en-us \
+    git \
+    curl \
+    unzip \
+    locales \
+    sudo \
+    # For health checks and debugging
+    netcat-openbsd \
+    gnupg \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install MariaDB client from Debian repositories
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends mariadb-client \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
+
+# Set up locales
+RUN sed -i 's/^# en_US.UTF-8 /en_US.UTF-8 /' /etc/locale.gen && locale-gen
+
+# Install PHP extensions
+# Note: php-imap is NOT required - Roundcube has its own IMAP implementation using sockets
+# Note: pspell is optional - Roundcube can use enchant or web-based spellcheck (googie) instead
+RUN docker-php-ext-configure gd --with-jpeg --with-freetype \
+    && docker-php-ext-configure ldap \
+    && docker-php-ext-install \
+        zip \
+        pcntl \
+        gd \
+        ldap \
+        intl \
+        enchant \
+        pdo_mysql \
+        pdo_pgsql \
+        pdo_sqlite \
+        mysqli \
+        opcache \
+        exif
+
+# Install Imagick via PECL (for image manipulation)
+RUN pecl install imagick && docker-php-ext-enable imagick
+
+# Install Xdebug for debugging
+RUN pecl install xdebug && docker-php-ext-enable xdebug
+
+# Configure Xdebug
+RUN echo "xdebug.mode=debug,coverage" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
+    && echo "xdebug.start_with_request=trigger" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini \
+    && echo "xdebug.client_host=host.docker.internal" >> /usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini
+
+# Configure PHP for development (increase memory limit for tools like PHPStan)
+RUN echo "memory_limit=512M" >> /usr/local/etc/php/conf.d/dev-settings.ini
+
+# Install Composer
+COPY --from=composer:2 /usr/bin/composer /usr/bin/composer
+
+# Create non-root user
+ARG USERNAME=vscode
+ARG USER_UID=1000
+ARG USER_GID=$USER_UID
+
+RUN groupadd --gid $USER_GID $USERNAME \
+    && useradd --uid $USER_UID --gid $USER_GID -m $USERNAME \
+    && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$USERNAME \
+    && chmod 0440 /etc/sudoers.d/$USERNAME
+
+# Set working directory
+WORKDIR /workspace
+
+USER $USERNAME

.devcontainer/Dockerfile.dovecot

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+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    dovecot-core \
+    dovecot-imapd \
+    dovecot-lmtpd \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create test user (password: testpass)
+RUN useradd -m -s /bin/bash testuser \
+    && echo "testuser:testpass" | chpasswd
+
+# Create mail directories
+RUN mkdir -p /var/mail/testuser \
+    && chown -R testuser:testuser /var/mail/testuser
+
+# Configure Dovecot (use 99-* to override Debian defaults)
+RUN echo 'protocols = imap lmtp' > /etc/dovecot/conf.d/99-custom.conf \
+    && echo 'mail_location = maildir:/var/mail/%u' >> /etc/dovecot/conf.d/99-custom.conf \
+    && echo 'disable_plaintext_auth = no' >> /etc/dovecot/conf.d/99-custom.conf \
+    && echo 'auth_mechanisms = plain login' >> /etc/dovecot/conf.d/99-custom.conf \
+    && echo 'ssl = no' >> /etc/dovecot/conf.d/99-custom.conf \
+    && echo 'listen = *' >> /etc/dovecot/conf.d/99-custom.conf \
+    && printf 'service lmtp {\n  inet_listener lmtp {\n    port = 24\n  }\n}\n' >> /etc/dovecot/conf.d/99-custom.conf
+
+# Create entrypoint script to initialize maildir at runtime (after volume mount)
+# In maildir format, the root directory IS the INBOX (no .INBOX subfolder)
+RUN printf '#!/bin/bash\n\
+if [ ! -d /var/mail/testuser/cur ]; then\n\
+    mkdir -p /var/mail/testuser/{cur,new,tmp}\n\
+    chown -R testuser:testuser /var/mail/testuser\n\
+fi\n\
+exec "$@"\n' > /entrypoint.sh \
+    && chmod +x /entrypoint.sh
+
+EXPOSE 143 24
+
+ENTRYPOINT ["/entrypoint.sh"]
+CMD ["dovecot", "-F"]