Adding Dockerfile for compiling cross and kernel

Author: chabala

.dockerignore

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+.git
+.github
+Documentation
+Screenshots
+

Dockerfile

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+# suggested usage: docker build --rm -t elks-dist:latest . && docker run -it --name elks-dist elks-dist:latest
+FROM ubuntu:20.04
+ENTRYPOINT ["bash"]
+# install required tools, make non-root user and switch to it
+ENV USER=builder \
+    UID=1000 \
+    GID=1000 \
+    DEBIAN_FRONTEND=noninteractive \
+    DEBCONF_NONINTERACTIVE_SEEN=true
+WORKDIR /elks
+RUN apt-get update -qq \
+ && apt-get install -y --no-install-recommends \
+  flex bison texinfo libncurses5-dev \
+  bash make g++ git libelf-dev \
+  xxd ca-certificates wget mtools \
+ && rm -r /var/lib/apt/lists /var/cache/apt/archives \
+ && addgroup \
+    --gid $GID \
+    "$USER" \
+ && adduser \
+    --disabled-password \
+    --gecos "" \
+    --home "/elks" \
+    --ingroup "$USER" \
+    --no-create-home \
+    --uid "$UID" \
+    "$USER" \
+ && chown $UID:$GID /elks
+USER $USER
+# copy in code and build cross tooling
+COPY --chown=$USER:$USER . /elks
+RUN mkdir -p "cross" \
+ && tools/build.sh
+
+# run the rest of the build interactively from step 3: https://github.com/jbruchon/elks/blob/master/BUILD.md
+# . ./env.sh
+# make menuconfig
+# make all
+
+# tarball the results and copy them out of the container
+# cd image && tar -cvzf binfiles.tar.gz *.bin
+
+# outside the container, on the host
+# docker cp elks-dist:/elks/image/binfiles.tar.gz .