avoid sbt rebuilding if just scala upgrades

image-amazon/Dockerfile

@@ -2,7 +2,6 @@
 FROM amazoncorretto:8u222
 
 # The following is inspired by https://github.com/hseeberger/scala-sbt/blob/master/oracle/Dockerfile
-ENV SCALA_VERSION 2.13.1
 ENV SBT_VERSION 1.3.0
 RUN \
   curl -L -o /root/sbt-${SBT_VERSION}.rpm https://bintray.com/sbt/rpm/download_file?file_path=sbt-${SBT_VERSION}.rpm && \
@@ -11,6 +10,7 @@ RUN \
   rm /root/sbt-${SBT_VERSION}.rpm
 
 # Prepare sbt and Scala, verify compilation including JavaFX classes
+ENV SCALA_VERSION 2.13.1
 WORKDIR /root/dummy
 RUN \
   mkdir -p project && \

image-debian/Dockerfile

@@ -5,14 +5,14 @@ FROM buildpack-deps:stretch-scm
 RUN apt-get update && apt-get install -y --no-install-recommends openjdk-8-jdk openjfx && rm -rf /var/lib/apt/lists/*
 
 # The following is inspired by https://github.com/hseeberger/scala-sbt/blob/master/debian/Dockerfile
-ENV SCALA_VERSION 2.13.1
 ENV SBT_VERSION 1.3.0
 RUN \
   curl -L -o /root/sbt-${SBT_VERSION}.deb https://dl.bintray.com/sbt/debian/sbt-${SBT_VERSION}.deb && \
   apt-get install -y /root/sbt-${SBT_VERSION}.deb && \
   rm /root/sbt-${SBT_VERSION}.deb
 
 # Prepare sbt and Scala, verify compilation including JavaFX classes
+ENV SCALA_VERSION 2.13.1
 WORKDIR /root/dummy
 RUN \
   mkdir -p project && \

image-openjdk/Dockerfile

@@ -5,7 +5,6 @@ FROM openjdk:8u222-jdk-stretch
 RUN apt-get update && apt-get install -y --no-install-recommends openjfx && rm -rf /var/lib/apt/lists/*
 
 # The following is inspired by https://github.com/hseeberger/scala-sbt/blob/master/debian/Dockerfile
-ENV SCALA_VERSION 2.13.0
 ENV SBT_VERSION 1.3.0
 RUN \
   curl -L -o /root/sbt-${SBT_VERSION}.deb https://dl.bintray.com/sbt/debian/sbt-${SBT_VERSION}.deb && \
@@ -18,6 +17,7 @@ RUN \
   ln -s -t "$JAVA_HOME/jre/lib/ext/" $(find /usr/share/java/openjfx/jre/lib/ext/ -maxdepth 1 -type f)
 
 # Prepare sbt and Scala, verify compilation including JavaFX classes
+ENV SCALA_VERSION 2.13.1
 WORKDIR /root/dummy
 RUN \
   mkdir -p project && \