add a linux-micro image to use locally and during tests

Author: pietroalbini

Diff for: linux-micro/Dockerfile

@@ -0,0 +1,9 @@
+FROM ubuntu:focal
+
+# Install the packages contained in `packages.txt`
+COPY packages.txt /opt/crates-build-env/packages.txt
+RUN apt-get update && \
+    cat /opt/crates-build-env/packages.txt | DEBIAN_FRONTEND=noninteractive xargs apt-get install -y && \
+    rm -rf /var/lib/apt/lists/*
+
+CMD ["bash"]

Diff for: linux-micro/packages.txt

@@ -0,0 +1,2 @@
+build-essential
+libssl-dev