Add a Dockerfile for container image building

Dockerfile

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+# Temporary image to download the package
+FROM maven:3.8 as builder
+
+ADD . /build
+WORKDIR /build
+
+RUN apt-get update \
+    && apt install -y python3 \
+    && ln -s /usr/bin/python3 /usr/bin/python
+RUN python3 tools/createCompletionLists.py && mvn verify \
+    && VERSION=$(grep "<version>.*</version>" pom.xml | head -n1 | cut -d '>' -f 2 | cut -d '<' -f 1) \
+    && cd target \
+    && tar xvf ltex-ls-${VERSION}.tar.gz \
+    && mv ltex-ls-${VERSION} /app \
+    && rm /app/bin/*.bat \
+    && find /app -maxdepth 1 -type f -not -name LICENSE.md -delete
+
+# Actual image
+FROM openjdk:slim
+
+ENV PATH="${PATH}:/app/bin"
+WORKDIR /app/bin
+COPY --from=builder /app /app
+CMD [ "echo", "Usage: 'ltex-ls [PARAMS]' or 'ltex-cli [PARAMS]'. A directory must be mounted to analyse files using ltex-cli." ]