Speed up docker builds (#1767)

* Copy over `requirements.txt` first in Dockerfile to cache Python dependencies

Dockerfile

@@ -14,9 +14,12 @@ RUN apt-get update \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
-COPY . /opt/CTFd
+COPY requirements.txt /opt/CTFd/
 
 RUN pip install -r requirements.txt --no-cache-dir
+
+COPY . /opt/CTFd
+
 # hadolint ignore=SC2086
 RUN for d in CTFd/plugins/*; do \
         if [ -f "$d/requirements.txt" ]; then \