Move CI management from lampepfl/dotty-ci (#19697)

Instead of managing the Docker image in
[lampepfl/dotty-ci](https://github.com/lampepfl/dotty-ci), we can move
the `Dockerfile` to this repository and manage it as a single monorepo.

I've placed the `Dockerfile` under the `.github` as it is part of the
"GitHub infrastructure".

.github/Dockerfile

@@ -0,0 +1,25 @@
+FROM ubuntu:22.04
+
+# The default locale is "POSIX" which is just ASCII.
+ENV LANG C.UTF-8
+ENV DEBIAN_FRONTEND noninteractive
+ENV TZ Europe/Zurich
+
+# Add packages to image, set default JDK version
+RUN apt-get update && \
+    apt-get upgrade -y && \
+    # Use a PPA to get Java 17
+    apt-get install -y software-properties-common && add-apt-repository ppa:openjdk-r/ppa && \
+    apt-get install -y bash curl git ssh htop nano vim-tiny zile \
+                       openjdk-8-jdk-headless \
+                       openjdk-17-jdk-headless \
+                       openjdk-21-jdk-headless && \
+    (curl -fsSL https://deb.nodesource.com/setup_18.x | bash -) && \
+    apt-get install -y nodejs
+
+
+# Install sbt
+ENV SBT_HOME /usr/local/sbt
+ENV PATH ${SBT_HOME}/bin:${PATH}
+ENV SBT_VERSION 1.9.0
+RUN curl -sL "https://github.com/sbt/sbt/releases/download/v$SBT_VERSION/sbt-$SBT_VERSION.tgz" | gunzip | tar -x -C /usr/local