Document packages for installing Canvas via Docker

Signed-off-by: Matt Cengia <mattcen@mattcen.com>

Dockerfile

@@ -10,6 +10,8 @@ ARG JS_SDK_REPO="https://github.com/matrix-org/matrix-js-sdk.git"
 ARG JS_SDK_BRANCH="master"
 
 RUN apt-get update && apt-get install -y git dos2unix \
+# These packages are required for building Canvas on architectures like Arm
+# See https://www.npmjs.com/package/canvas#compiling
   build-essential libcairo2-dev libpango1.0-dev libjpeg-dev libgif-dev librsvg2-dev
 
 WORKDIR /src