Add Mbed GCC 9 CI image

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