Update Dockerfile for better compatibility

This includes:
- Installing more recent Gradle
- Running setup commands in a bash login shell
- Pinning the Poetry version
- a few other minor tweaks

Dockerfile

@@ -9,10 +9,7 @@ ENV PROJECT_DIR=/project/
 # Expecting volume mount at
 ENV GATORGRADER_DIR=/root/.local/share/
 
-# Set Python version
-ARG PYTHON_VERSION='3.8.5'
-
-# Configure environment variables for Python
+# Configure environment variables
 ENV PYTHONUNBUFFERED=1 \
     # Prevent Python from creating .pyc files
     PYTHONDONTWRITEBYTECODE=1 \
@@ -36,29 +33,37 @@ ENV PYTHONUNBUFFERED=1 \
     # Specify the paths for using requirements and virtual environments;
     # this is where the requirements + virtual environment will live
     PYSETUP_PATH="/opt/pysetup" \
-    VENV_PATH="/opt/pysetup/.venv"
+    VENV_PATH="/opt/pysetup/.venv" \
+    # Specify the version of Gradle
+    GRADLE_VERSION=5.4.1 \
+    # Specify the home directory of Gradle
+    GRADLE_HOME="/opt/gradle-5.4.1"
 
 # Pre-pend Poetry's home and the .venv directory to PATH
-ENV PATH="$POETRY_HOME/bin:$VENV_PATH/bin:$PATH:"
+# Pre-pend Gradle's bin to PATH
+ENV PATH="$POETRY_HOME/bin:$VENV_PATH/bin:$GRADLE_HOME/bin:$PATH"
 
 # Define the project directory as the working directory
 WORKDIR ${PROJECT_DIR}
 
 # Specify shared volume storage in the container
 VOLUME ${PROJECT_DIR} ${GATORGRADER_DIR}
 
-# hadolint ignore=DL3008,DL3013,DL3015,DL3016,DL3018,DL3028
-RUN set -ex && echo "Installing Packages with apt-get..." \
+# Tell Docker to use a bash login shell for RUN commands
+SHELL ["/bin/bash", "--login", "-c"]
+
+# hadolint ignore=DL3008,DL3013,DL3016,DL3018,DL3028
+RUN set -e && echo "Installing Packages with apt-get..." \
     && export DEBIAN_FRONTEND=noninteractive \
     && apt-get update \
-    && apt-get -y install --no-install-recommends bash git ruby openjdk-11-jdk gradle npm \
+    && apt-get -y install --no-install-recommends ruby openjdk-11-jdk npm \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/* \
     && echo "Installing Pandoc..." \
     && wget -O /pandoc.tar.gz https://github.com/jgm/pandoc/releases/download/2.10.1/pandoc-2.10.1-linux-amd64.tar.gz \
     && tar -C /usr --strip-components 1 -xzvf /pandoc.tar.gz && rm /pandoc.tar.gz \
     && echo "Testing Pandoc..." \
-    && /usr/bin/pandoc --version \
+    && pandoc --version \
     && echo "Installing Markdown Linter called mdl..." \
     && gem install mdl \
     && echo "Installing HTML Linter called htmlhint..." \
@@ -69,12 +74,17 @@ RUN set -ex && echo "Installing Packages with apt-get..." \
     && echo "Testing Pip..." && pip --version \
     && echo "Installing Poetry..." \
     && wget -O /get-poetry.py https://raw.githubusercontent.com/sdispater/poetry/master/get-poetry.py \
-    && python /get-poetry.py && rm /get-poetry.py \
+    && python /get-poetry.py --version $POETRY_VERSION && rm /get-poetry.py \
+    && source $POETRY_HOME/env \
     && echo "Testing Poetry..." && poetry --version \
     && echo "Installing Pipenv..." \
     && pip install pipenv \
     && echo "Testing Pipenv..." && pipenv --version \
+    && echo "Installing Gradle..." \
+    && wget -O /gradle-bin.zip "https://services.gradle.org/distributions/gradle-${GRADLE_VERSION}-bin.zip" \
+    && unzip /gradle-bin.zip -d /opt && rm /gradle-bin.zip \
     && echo "Configuring Gradle..." \
+    && export PATH="$GRADLE_HOME/bin:$PATH" \
     && mkdir -p /root/.gradle/ \
     && echo "org.gradle.daemon=true" >> /root/.gradle/gradle.properties \
     && echo "systemProp.org.gradle.internal.launcher.welcomeMessageEnabled=false" >> /root/.gradle/gradle.properties \