add gcc so markupsafe compiles speedups

Author: davidism

Diff for: compose/gettingstarted.md

@@ -86,6 +86,7 @@ following:
     WORKDIR /code
     ENV FLASK_APP app.py
     ENV FLASK_RUN_HOST 0.0.0.0
+    RUN apk add --no-cache gcc musl-dev linux-headers
     COPY requirements.txt requirements.txt
     RUN pip install -r requirements.txt
     COPY . .
@@ -96,6 +97,7 @@ This tells Docker to:
 * Build an image starting with the Python 3.7 image.
 * Set the working directory to `/code`.
 * Set environment variables used by the `flask` command.
+* Install gcc so Python packages such as MarkupSafe and SQLAlchemy can compile speedups.
 * Copy `requirements.txt` and install the Python dependencies.
 * Copy the current directory `.` in the project to the workdir `.` in the image.
 * Set the default command for the container to `flask run`.