feat: user multi-stage builds and remove apt and pip caches

1. Remove apt cache after running `apt-get`
2. Pass `--no-cache-dir` to the `pip` command
3. Use multi-stage builds to remove the test layers from the final
   image.

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

feat: use chown during copy to reduce image size and build time

Using the `--chown` flag during `COPY` is much faster than running
`chown -R`. It also removes a layer from the image, resulting in faster
and smaller builds.

Resolves #55

Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

Remove armhf templates and move to multi-arch / buildx

Migrate as per the python3 template in the classic Go
templates, where a separate -armhf/-arm64 template is not
required and TARGETPLATFORM is introduced for use with
buildkit / buildx and faas-cli publish.

Tested with an RPi with a 32-bit OS and with a 64-bit ARM
OS (Ubuntu 20.04).

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Update examples in README and put http template first

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Bump watchdog to 0.4.0

Signed-off-by: Martin Dekov (VMware) <mdekov@vmware.com>