Add dockerfile and fix dependencies

- One dependency was missing
- Dockerfile simplifies creating an environment where this script can run

Dockerfile

@@ -0,0 +1,16 @@
+FROM alpine:3.10
+
+WORKDIR /code
+
+COPY requirements.txt /code/requirements.txt
+
+RUN apk add --no-cache --virtual py-build-deps gcc libc-dev libxslt-dev python3-dev && \
+    apk add --no-cache python3 libxslt && \
+    pip3 install -r requirements.txt && \
+    apk del py-build-deps
+
+# Copy this after installing since it changes the most frequently, to take
+# advantage of the image layer caching.
+COPY dl.py /code/dl.py
+
+ENTRYPOINT ["python3", "dl.py"]

requirements.txt

@@ -1,3 +1,4 @@
 opml
 feedparser
 youtube-dl
+python-dateutil