Multiarch image builds for Docker

[Snawoot]

Closes #4331

This PR contains following changes:

• linux/amd64, linux/arm64 and linux/arm/v7 multiarch image builds for Docker.
• Upgrade Alpine base image in order to fix CVE-2021-28831.
• Little cleanup: removed old dangling docker build scripts, consolidated code.

arm/v7 image was tested on Docker running on RPi 3B+

[codecov-commenter]

Codecov Report

Merging #4346 (4f5c710) into master (293fe8b) will increase coverage by 0.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4346      +/-   ##
==========================================
+ Coverage   40.92%   41.07%   +0.15%     
==========================================
  Files         334      334              
  Lines       17965    17869      -96     
==========================================
- Hits         7352     7340      -12     
+ Misses       9919     9834      -85     
- Partials      694      695       +1     

Impacted Files	Coverage Δ
ci/release/docker.go	0.00% <0.00%> (ø)
session/pingpong/invoice_tracker.go	62.76% <0.00%> (-2.41%)	⬇️
core/discovery/discovery.go	63.49% <0.00%> (-1.59%)	⬇️
requests/dialer_swarm.go	75.93% <0.00%> (-1.51%)	⬇️
nat/traversal/pinger.go	77.51% <0.00%> (-0.39%)	⬇️
tequilapi/endpoints/sse_handler.go	71.32% <0.00%> (+0.73%)	⬆️

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