Dockerfile: Install python3-distutils

The build process fails without that.

Co-authored-by: Tom Herbers <mail@tomherbers.de>

contrib/docker/Dockerfile

@@ -6,6 +6,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     file \
     git \
     python3 \
+    python3-distutils \
     build-essential \
     gawk \
     unzip \

docs/user/getting_started.rst

@@ -29,6 +29,7 @@ freshly installed Debian Bullseye system the following packages are required:
 
 * `git` (to get Gluon and other dependencies)
 * `python3`
+* `python3-distutils`
 * `build-essential`
 * `ecdsautils` (to sign firmware, see `contrib/sign.sh`)
 * `gawk`