Nvidia Jetson ffmpeg + TensorRT support (#6458)

* Non-Jetson changes

Required for later commits:
- Allow base image to be overridden (and don't assume its WORKDIR)
- Ensure python3.9
- Map hwaccel decode presets as strings instead of lists
Not required:
- Fix existing documentation
- Simplify hwaccel scale logic

* Prepare for multi-arch tensorrt build

* Add tensorrt images for Jetson boards

* Add Jetson ffmpeg hwaccel

* Update docs

* Add CODEOWNERS

* CI

* Change default model from yolov7-tiny-416 to yolov7-320

In my experience the tiny models perform markedly worse without being
much faster

* fixup! Update docs

docker/main/install_deps.sh

@@ -10,11 +10,15 @@ apt-get -qq install --no-install-recommends -y \
     wget \
     procps vainfo \
     unzip locales tzdata libxml2 xz-utils \
+    python3.9 \
     python3-pip \
     curl \
     jq \
     nethogs
 
+# ensure python3 defaults to python3.9
+update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 1
+
 mkdir -p -m 600 /root/.gnupg
 
 # add coral repo
@@ -23,8 +27,10 @@ curl -fsSLo - https://packages.cloud.google.com/apt/doc/apt-key.gpg | \
 echo "deb https://packages.cloud.google.com/apt coral-edgetpu-stable main" | tee /etc/apt/sources.list.d/coral-edgetpu.list
 echo "libedgetpu1-max libedgetpu/accepted-eula select true" | debconf-set-selections
 
-# enable non-free repo
-sed -i -e's/ main/ main contrib non-free/g' /etc/apt/sources.list
+# enable non-free repo in Debian
+if grep -q "Debian" /etc/issue; then
+    sed -i -e's/ main/ main contrib non-free/g' /etc/apt/sources.list
+fi
 
 # coral drivers
 apt-get -qq update