ISSUE-238 Improved build steps in Dockerfile to allow caching and improved docker instructions

.dockerignore

@@ -1,4 +1,6 @@
 .git/
 .vscode/
+screenshot/
+tests/
 *.txt
 !/requirements.txt

Dockerfile

@@ -1,20 +1,27 @@
 FROM python:3.7-alpine as build
-RUN apk add --no-cache linux-headers g++ gcc libxml2-dev libxml2 libxslt-dev
-COPY requirements.txt /opt/sherlock/
 WORKDIR /wheels
+RUN apk update --no-cache \
+  && apk add --no-cache \
+    g++ \
+    gcc \
+    libxml2 \
+    libxml2-dev \
+    libxslt-dev \
+    linux-headers
+COPY requirements.txt /opt/sherlock/
 RUN pip3 wheel -r /opt/sherlock/requirements.txt
 
-FROM python:3.7-alpine
-COPY --from=build /wheels /wheels
-COPY . /opt/sherlock/
-WORKDIR /opt/sherlock
-RUN pip3 install -r requirements.txt -f /wheels \
-  && rm -rf /wheels \
-  && rm -rf /root/.cache/pip/*
 
+FROM python:3.7-alpine
+WORKDIR /opt/sherlock
 ARG VCS_REF
 ARG VCS_URL="https://github.com/sherlock-project/sherlock"
 LABEL org.label-schema.vcs-ref=$VCS_REF \
       org.label-schema.vcs-url=$VCS_URL
+COPY --from=build /wheels /wheels
+COPY . /opt/sherlock/
+RUN pip3 install -r requirements.txt -f /wheels \
+  && rm -rf /wheels \
+  && rm -rf /root/.cache/pip/*
 
 ENTRYPOINT ["python", "sherlock.py"]

README.md

@@ -103,13 +103,22 @@ docker build -t mysherlock-image .
 Once the image is built sherlock can be invoked by running the following:
 
 ```
-docker run --rm mysherlock-image user123
+docker run --rm -t mysherlock-image user123
 ```
 
-The optional ```--rm``` flag removes the container filesystem on completion to prevent cruft build-up.  See https://docs.docker.com/engine/reference/run/#clean-up---rm
+The optional ```--rm``` flag removes the container filesystem on completion to prevent cruft build-up. See https://docs.docker.com/engine/reference/run/#clean-up---rm
 
-One caveat is the text file that is created will only exist in the container so you will not be able to get at that.
+The optional ```-t``` flag allocates a pseudo-TTY which allows colored output. See https://docs.docker.com/engine/reference/run/#foreground
 
+It is possible to use the following command to access the saved results:
+
+```
+docker run --rm -t -v "$PWD/results:/opt/sherlock/results" mysherlock-image -o /opt/sherlock/results/text.txt user123
+```
+
+The ```-v "$PWD/results:/opt/sherlock/results"``` option tells docker create (or use) the folder `results` in the
+present working directory and to mount it at `/opt/sherlock/results` on the docker container.
+The `-o /opt/sherlock/results/text.txt` option tells `sherlock` to output the result
 
 Or you can simply use "Docker Hub" to run `sherlock`:
 ```