This PR overhauls the Dockerfile.custom build process to support cross-architecture builds and significantly optimizes build performance.

[nihaldivyam]

Build Performance & Context

• Optimized Build Context: Created a .dockerignore file to exclude node_modules, .parcel-cache, and other local build artifacts. This reduced the build context transfer
  from 1.44GB to ~1MB, dramatically speeding up the initial build phase.
• Improved Layer Caching:
  • Restructured the webapp builder to copy workspace package.json files individually before running npm ci, allowing for better Docker layer caching when source files
    change.
  • Restructured the server builder to copy go.work and the public module's dependency files before downloading modules.

Native Dependency & Architecture Support

• Node.js Environment: Switched the webapp-builder to Node 22 (LTS) for better compatibility with native modules like canvas on modern V8 versions.
• System Libraries: Added required Debian libraries (libcairo2-dev, libpango1.0-dev, libjpeg-dev, pkg-config, etc.) to the builder stage to ensure native modules can be
  compiled for Linux ARM64.
• Native Module Handling:
  • Implemented npm ci --ignore-scripts followed by a targeted npm rebuild canvas to ensure native bindings are correct for the target OS without triggering
    problematic image optimization binaries.
• Go Build Fixes:
  • Updated Go version to 1.26.1 to satisfy go.work requirements.
  • Fixed the go build target to ./cmd/mattermost to resolve "multiple packages" errors and ensure a single binary output.

Webapp Optimization

• Optional Image Optimization: Modified webapp/channels/webpack.config.js to support the SKIP_IMAGE_OPTIMIZATION environment variable. This allows the Docker build to
  bypass image-webpack-loader (which often fails on ARM64 due to incompatible pre-built binaries for optipng and gifsicle) while maintaining a production-ready
  configuration.

Verification Results

• Successfully built the Mattermost production image on an ARM64 host (Mac Apple Silicon) targeting Linux ARM64.
• Verified that both the server and mmctl binaries are correctly built and placed in the final image.
• Verified that webapp assets are correctly compiled and copied to the /mattermost/client directory.