Include original 18.04 steps

Author: steved0x

docs/pipelines/agents/docker.md

@@ -202,36 +202,70 @@ Next, create the Dockerfile.
     ```
 
 4. Save the following content to `~/dockeragent/Dockerfile`:
-
-    ```docker
-    FROM ubuntu:20.04
-
-    RUN DEBIAN_FRONTEND=noninteractive apt-get update
-    RUN DEBIAN_FRONTEND=noninteractive apt-get upgrade -y
-
-    RUN DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends \
-        apt-transport-https \
-        apt-utils \
-        ca-certificates \
-        curl \
-        iputils-ping \
-        jq \
-        lsb-release \
-        software-properties-common
-
-    RUN curl -sL https://aka.ms/InstallAzureCLIDeb | bash
-
-    # Can be 'linux-x64', 'linux-arm64', 'linux-arm', 'rhel.6-x64'.
-    ENV TARGETARCH=linux-x64
-
-    WORKDIR /azp
-
-    COPY ./start.sh .
-    RUN chmod +x start.sh
-
-    ENTRYPOINT [ "./start.sh" ]
-    ```
-
+    * For Ubuntu 20.04:
+      ```docker
+      FROM ubuntu:20.04
+
+      RUN DEBIAN_FRONTEND=noninteractive apt-get update
+      RUN DEBIAN_FRONTEND=noninteractive apt-get upgrade -y
+
+      RUN DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends \
+          apt-transport-https \
+          apt-utils \
+          ca-certificates \
+          curl \
+          iputils-ping \
+          jq \
+          lsb-release \
+          software-properties-common
+
+      RUN curl -sL https://aka.ms/InstallAzureCLIDeb | bash
+
+      # Can be 'linux-x64', 'linux-arm64', 'linux-arm', 'rhel.6-x64'.
+      ENV TARGETARCH=linux-x64
+
+      WORKDIR /azp
+
+      COPY ./start.sh .
+      RUN chmod +x start.sh
+
+      ENTRYPOINT [ "./start.sh" ]
+      ```
+  * For Ubuntu 18.04:
+      ```docker
+      FROM ubuntu:18.04
+
+      # To make it easier for build and release pipelines to run apt-get,
+      # configure apt to not require confirmation (assume the -y argument by default)
+      ENV DEBIAN_FRONTEND=noninteractive
+      RUN echo "APT::Get::Assume-Yes \"true\";" > /etc/apt/apt.conf.d/90assumeyes
+
+      RUN apt-get update && apt-get install -y --no-install-recommends \
+          ca-certificates \
+          curl \
+          jq \
+          git \
+          iputils-ping \
+          libcurl4 \
+          libicu60 \
+          libunwind8 \
+          netcat \
+          libssl1.0 \
+        && rm -rf /var/lib/apt/lists/*
+
+      RUN curl -LsS https://aka.ms/InstallAzureCLIDeb | bash \
+        && rm -rf /var/lib/apt/lists/*
+
+      # Can be 'linux-x64', 'linux-arm64', 'linux-arm', 'rhel.6-x64'.
+      ENV TARGETARCH=linux-x64
+
+      WORKDIR /azp
+
+      COPY ./start.sh .
+      RUN chmod +x start.sh
+
+      ENTRYPOINT ["./start.sh"]
+      ```
    > [!NOTE]
    > Tasks might depend on executables that your container is expected to provide.
    > For instance, you must add the `zip` and `unzip` packages