changed docker-image to alpine

[Cyber1000]

• alpine might be more lightweigth than a ubuntu-based-container
  • used suggestion from Alternative Dockerfile #796, but made some adjustments:
    • don't use alpine:latest, but alpine:3.12, since it's better to use fixed versions
    • some packages where still missing
• updated readme:
  • added some troubleshooting information, like firewall problems: [Suggestion] NixOS Firewall Troubleshooting #638
  • or how to get the name of your interface: How to connect USB WiFi adapter when using Docker? #793
• since it's not based on phusion/baseimage-docker anymore bug Docker exits unexpectedly #840 won't occur again
• In this image I don't separate between building and starting the image, it should be easier to use for the average user. In general I did some cleanup to make usage easier. This was a suggestion in Changing wlan device in docker not working #532 (comment) -> in general Changing wlan device in docker not working #532 shouldn't occur in the future, since I refactored the docker-part.
• Added iproute2 iputils-ping to install_prereq.sh since that is already needed and normally installed on ubuntu/debian, but getting into problems if not

The issues I linked here may be closed after merging, since all relevant info is said and/or problems got fixed

[Cyber1000]

This may also fix (or at least make troubleshooting easier) - whoever had this problems, please check after merging:

• Cannot build docker Tuya-convert #811
• vtrust-flash doesn't turn up despite of successful report #900
• Docker issue, possibly "address already in use" #570

[Cyber1000]

Note to myself:

• this image should work with podman - testing myself on fedora, if positive PR adds podman support #732 may be obsolete (@ohadlevy may test this too after merging this PR)
• this PR adds a switch for different OSes: Arch Linux support for install_prereq.sh #890 - After this is merged I would add a switch for alpine OS, to get everything for setting up a host or docker into install_prereq.sh. For now the setup is kept within Dockerfile. To make the correct switch-statement: The ID is alpine in /etc/os-release

[Daveyr]

This might also fix #922, where I can't use the base image on a Raspberry Pi 4. Alpine supports armv7 so in theory it should run fine.

[kueblc]

@Cyber1000 @ohadlevy @aravindkoneru

Thank you all for your contributions! I apologize for the delay getting back to this.

Since I am not at all familiar with Docker, can you please let me know which of #732, #915, #923 is ready for merge and which are obsolete/redundant?

[Cyber1000]

Sorry my time was limited too due to heavy work load

#923 -> does exactly the same as this PR (#915), but #915 does a little bit more (especially using alpine as base which can be used on raspberry os too)
#732 -> uses fedora-minimal which won't work on raspberry os. I haven't tested my new docker on fedora (no time/no WLAN-Interface on my fedora machine), so someone may try it. Anyway it it safe to merge #732, it just is an alternative Dockerfile named Dockerfile.fedora, which doesn't get used automatically, you would have to include it in your yml (or overwrite the original Dockerfile) to use it - we may add something to the README if we merge this

[kueblc]

Sorry for the delay! Day jobs...