Docker improvements (#117)

Dockerfile

@@ -1,7 +1,13 @@
-FROM python:3.8.7-alpine
+FROM python:3.9-alpine
 
-RUN apk --no-cache add exiftool curl \
+RUN  apk add --no-cache --virtual .build-dependencies \
+        curl \
+    && apk --no-cache add exiftool \
     && curl -L https://github.com/ivandokov/phockup/archive/latest.tar.gz -o phockup.tar.gz \
     && tar -zxf phockup.tar.gz \
     && mv phockup-* /opt/phockup \
-    && ln -s /opt/phockup/phockup.py /usr/local/bin/phockup
+    && ln -s /opt/phockup/phockup.py /usr/local/bin/phockup \
+    && apk del --no-cache --purge .build-dependencies \
+    && rm phockup.tar.gz
+
+ENTRYPOINT [ "phockup" ]

readme.md

@@ -52,7 +52,7 @@ brew install phockup
 
 ### Docker
 ```
-docker run -v ~/Pictures:/mnt ivandokov/phockup:latest phockup /mnt/Input /mnt/Output [PHOCKUP ARGUMENTS]
+docker run -v ~/Pictures:/mnt ivandokov/phockup:latest /mnt/Input /mnt/Output [PHOCKUP ARGUMENTS]
 ```
 
 The `-v ~/Pictures:/mnt` part of the command mounts your `~/Pictures` directory to `/mnt` inside the container. You can pass any **absolute** path to be mounted to the container and later on be used as paths for the `phockup` command. The example above provides your `~/Pictures/Input` as `INPUTDIR` and `~/Pictures/Output` as `OUTPUDIR`. You can pass additional arguments afterwards.