Add EXTRA_FLAGS for extended flag support

[bbugh]

This adds a new environment variable EXTRA_FLAGS so we can extend the existing flags passed to Chromium instead of clobbering them all with FLAGS and trying to reproduce the whole list ourselves.

For example, for our IoT project we need --audio-buffer-size=2048 and --audio-output-channels=8, so we can do this:

services:
  browser:
    image: bh.cr/balenalabs/browser-<arch>
    environment:
      EXTRA_FLAGS: --audio-buffer-size=2048 --audio-output-channels=8

instead of having this monstrosity (which requires us to keep track of window sizes):

services:
  browser:
    image: bh.cr/balenalabs/browser-<arch>
    environment:
      FLAGS: >-
        --disable-features=TranslateUI
        --disable-component-extensions-with-background-pages
        --disable-background-networking
        --disable-sync
        --metrics-recording-only
        --disable-default-apps
        --no-default-browser-check
        --no-first-run
        --disable-backgrounding-occluded-windows
        --disable-renderer-backgrounding
        --disable-background-timer-throttling
        --force-fieldtrials=*BackgroundTracing/default/
        --window-size=1920,1080
        --window-position=0,0
        --autoplay-policy=no-user-gesture-required
        --noerrdialogs
        --disable-session-crashed-bubble
        --check-for-update-interval=31536000
        --disable-dev-shm-usage
        --enable-zero-copy
        --num-raster-threads=4
        --ignore-gpu-blocklist
        --enable-gpu-rasterization
        --audio-buffer-size=2048
        --audio-output-channels=8

[bbugh]

Force pushed change-type: minor amend to the message for the CI.