Add Mbed GCC 9 CI image

.dockerignore

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+.git
+README.md

Dockerfile

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+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV LANG=C.UTF-8
+ENV LC_ALL=C.UTF-8
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        binutils-arm-none-eabi \
+        ca-certificates \
+        curl \
+        gcc-arm-none-eabi \
+        git \
+        make \
+        mercurial \
+        patch \
+        python3 \
+        python3-pip \
+        python3-setuptools \
+        python3-wheel \
+        unzip \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN python3 -m pip install --no-cache-dir "mbed-cli==1.10.5"
+
+RUN arm-none-eabi-gcc --version \
+    && python3 --version \
+    && mbed --version
+
+WORKDIR /workspace

README.md

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+# docker-mbed-gcc9
+
+Reusable CI image for Mbed OS 5 firmware builds.
+
+Published image:
+
+```text
+git.tbeslan.ru/beslan/mbed-gcc9:mbed-os5
+```
+
+Installed tools:
+
+- Ubuntu 20.04
+- Python 3
+- `mbed-cli==1.10.5`
+- `gcc-arm-none-eabi` from Ubuntu 20.04 packages
+- `make`, `patch`, `git`, `mercurial`, `curl`, `unzip`
+
+Build and push:
+
+```sh
+docker build -t git.tbeslan.ru/beslan/mbed-gcc9:mbed-os5 .
+docker push git.tbeslan.ru/beslan/mbed-gcc9:mbed-os5
+```