Set ImageOS in Actions runner Dockerfiles

This allows Ruby to be installed on those
non-standard images.

Dockerfile.dapper

@@ -1,4 +1,5 @@
 FROM ubuntu:18.04
+ENV ImageOS=ubuntu20
 RUN apt-get update && apt-get install -y g++ git cmake libgtk-3-dev \ 
     libglvnd-dev libglu1-mesa-dev freeglut3-dev libasound2-dev
 WORKDIR ${DAPPER_SOURCE:-/source}

build/android/Dockerfile

@@ -1,4 +1,5 @@
 FROM mingc/android-build-box
+ENV ImageOS=ubuntu20
 RUN apt-get update
 RUN apt-get remove git -y
 RUN apt-get install software-properties-common -y

build/baremetalpi/Dockerfile

@@ -1,4 +1,5 @@
 FROM ubuntu:18.04
+ENV ImageOS=ubuntu20
 RUN apt-get update
 RUN apt-get install software-properties-common -y
 RUN add-apt-repository ppa:git-core/ppa -y

build/n3ds/Dockerfile

@@ -1,4 +1,5 @@
 FROM ubuntu:18.04
+ENV ImageOS=ubuntu20
 RUN apt-get update
 RUN apt-get install software-properties-common -y
 RUN add-apt-repository ppa:git-core/ppa -y

build/rpi/Dockerfile

@@ -1,4 +1,5 @@
 FROM ubuntu:18.04
+ENV ImageOS=ubuntu20
 RUN apt-get update
 RUN apt-get install software-properties-common -y
 RUN add-apt-repository ppa:git-core/ppa -y