Dockerfile overhaul (#1726)

Create entrypoint to orchestrate the build steps
Supported commands: make, ninja
Passes additional arguments to the make / ninja command at the end (like -jNN)
There is a shortcut to make -jNN by just specifying -jNN
Anything else will be directly executed (like getting a shell into the container with bash -li is still possible)

firmware/tools/docker-entrypoint.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+set -e # exit immediatelly on any failure
+
+build_make() {
+   cd ..
+   mkdir -p build
+   cd build
+	cmake ..
+   make "$@"
+   exit 0 # Report success :)
+}
+
+build_ninja() {
+   cd ..
+   mkdir -p build
+   cd build
+	cmake -G Ninja ..
+   ninja "$@"
+   exit 0 # Report success :)
+}
+
+if [ "$1" = 'make' ]; then # build the default (or specified) target with make
+   shift # remove the first item from $@ as we consumed it, we can then pass the rest on to make
+   build_make "$@"
+elif [[ $1 == -j* ]]; then # allow passing -j without typing make_default
+   # dont shift here as we wanna pass the -j
+   build_make "$@"
+elif [ "$1" = 'ninja' ]; then # build the default (or specified) target with ninja
+   shift # remove the first item from $@ as we consumed it, we can then pass the rest on to make
+   build_ninja "$@"
+fi
+
+exec "$@"