added docker image support

* added environment variable support
* added docker instructions
* added .env hint

README.md

@@ -94,6 +94,33 @@ installed - `make`.
 
 To install after building either copy the binary or execute `sudo make install`.
 
+### Using the docker image
+
+A pre-built docker  image is available, which you can  use to test the
+app without  installing it. You  need `docker-compose`. Copy  the file
+`docker-compose.yaml` to somewhere, cd to that directory and execute:
+
+```shell
+mkdir kleinanzeigen-backup
+USER_ID=$(id -u) GROUP_ID=$(id -g) OUTDIR=./kleinanzeigen-backup docker-compose run kleingebaeck -u XXX -v
+```
+
+`USER_ID` and  `GROUP_ID` needs to  be specified  so that you  are the
+owner of the created backups. The backup directory `OUTDIR` must exist
+prior to the execution, otherwise docker  will create it as root, then
+kleingebaeck will  fail. You may  also use a  `.env` file in  the same
+directory containing the variables, such as:
+
+```
+USER_ID=1000
+GROUP_ID=1000
+OUTDIR=./kleinanzeigen-backup
+```
+
+You may of course also modify the `docker-compose.yaml` to suit your needs.
+
+If you want to build the image yourself, use the supplied Dockerfile.
+
 ## Commandline options:
 
 ```
@@ -128,6 +155,13 @@ loglevel = verbose
 outdir = "test"
 ```
 
+## Environment Variables
+
+Kleingebaeck can also be configured using environment variables. Just prefix the config variables with `KLEINGEBAECK_` and put them to upper case. Eg:
+```shell
+% KLEINGEBAECK_OUTDIR=/backup kleingebaeck -v
+```
+
 ## Usage
 
 To setup the tool, you need to lookup your userid on