Change Dockerfile to comply with best practices

In order of decreasing significance:
* Move addition of all files later to avoid premature build cache
  invalidation
* Add separate instructions to copy over files needed earlier
* Change deprecated MAINTAINER instruction to LABEL maintainer
* Change ADD to COPY, as ADD apparently behaves weirdly in some cases
* Alphabetize dependencies for readability

Dockerfile

@@ -21,22 +21,30 @@
 #
 FROM alpine
 
-MAINTAINER www.evennia.com
+LABEL maintainer="www.evennia.com"
 
 # install compilation environment
-RUN apk update && apk add python py-pip python-dev py-setuptools gcc \
-musl-dev jpeg-dev zlib-dev bash py2-openssl procps
+RUN apk update && apk add bash gcc jpeg-dev musl-dev procps py-pip \
+py-setuptools py2-openssl python python-dev zlib-dev
 
-# add the project source
-ADD . /usr/src/evennia
+# add the files required for pip installation
+COPY ./setup.py /usr/src/evennia/
+COPY ./requirements.txt /usr/src/evennia/
+COPY ./evennia/VERSION.txt /usr/src/evennia/evennia/
+COPY ./bin /usr/src/evennia/bin/
 
 # install dependencies
 RUN pip install -e /usr/src/evennia --trusted-host pypi.python.org
 RUN pip install cryptography pyasn1 service_identity
 
+# add the project source; this should always be done after all
+# expensive operations have completed to avoid prematurely
+# invalidating the build cache.
+COPY . /usr/src/evennia
+
 # add the game source when rebuilding a new docker image from inside
 # a game dir
-ONBUILD ADD . /usr/src/game
+ONBUILD COPY . /usr/src/game
 
 # make the game source hierarchy persistent with a named volume.
 # mount on-disk game location here when using the container