Improve handling of ~/.docker/config.json

[hadmut]

Act version

0.2.89

Feature description

Hi,

act is reading ~/.docker/config.json
based on
#527

I had some trouble running act, issuing just

[Versuch/blubb] Could not load docker config: parsing config file (/home/hadmut/.docker/config.json): invalid auth configuration file
[Versuch/blubb] parsing config file (/home/hadmut/.docker/config.json): invalid auth configuration file

which gave me some headache, since all other docker-related programs smoothly and silently worked with this file, and it was correct json.

The reason was that I had a bug in my configuration skripts creating the config file, encoding the auth entry twice with base64,i.e.
instead of
"username:password" -> encode64 -> write into config.json
I had
"username:password" -> encode64 -> encode64 -> write into config.json
which is not visible with pure eye and took me some time to find.

However, I'd propose two improvements:

1. have a command line parameter allowing to use a file other than ~/.docker/config.json (or none at all)
2. improve error messages about parsing