Working image with SBT pre-cached.

Containerfile

@@ -1,5 +1,10 @@
 FROM docker.io/library/eclipse-temurin:21-jdk-alpine
 
+ARG SCALA_VERSION
+ENV SCALA_VERSION ${SCALA_VERSION:-3.4.0}
+ARG SBT_VERSION
+ENV SBT_VERSION ${SBT_VERSION:-1.9.9}
+
 # git is required for checkout
 # python3 is required for pre-commit
 # node is required for Forgejo checkout
@@ -14,11 +19,25 @@ RUN adduser -S -h /home/builder -s /bin/bash -G builder -u 1000 builder
 USER builder
 WORKDIR /home/builder
 
+# Coursier provides all Scala development requirements.
 RUN curl -fLo coursier https://github.com/coursier/launchers/raw/master/coursier
 RUN chmod +x coursier
-RUN ./coursier setup --yes --apps coursier,sbt,sbt-launcher,scalafmt,scalac
+RUN ./coursier setup --yes
 RUN rm ./coursier
 RUN mkdir -p /home/builder/.local/bin
 ENV PATH="${PATH}:/home/builder/.local/share/coursier/bin"
 ENV PATH="${PATH}:/home/builder/.local/bin"
+
+# Make Git not complain.
 RUN git config --global init.defaultBranch main
+
+# Warm up the SBT cache (taken from official SBT Docker image).
+RUN \
+  sbt sbtVersion && \
+  mkdir -p project && \
+  echo "scalaVersion := \"${SCALA_VERSION}\"" > build.sbt && \
+  echo "sbt.version=${SBT_VERSION}" > project/build.properties && \
+  echo "// force sbt compiler-bridge download" > project/Dependencies.scala && \
+  echo "case object Temp" > Temp.scala && \
+  sbt compile && \
+  rm -r project && rm build.sbt && rm Temp.scala && rm -r target