docker: Support pregenerated keys in docker private networks

[winder]

Summary

Docker changes:

• Detect participation keys at the /etc/algorand/keys directory.
• If a private network is going to be started, use that directory if there are keys.
• README updates describing how the feature is used.

#5699

Test Plan

Pregenerate keys with goal:
goal network pregen -t some_template.json -p pregen

Pregenerate keys with docker:

docker run --rm -it \
    -v /path/to/your/template.json:/etc/algorand/template.json \
    -v $(pwd)/pregen:/algod/pregen \
    --entrypoint "/node/bin/goal" \
    algorand/algod:nightly network pregen -t /etc/algorand/template.json -p /algod/pregen

Build the image (from root dir):

docker build \
    -t wwinder/algod:pregen-test  \
    --build-arg CHANNEL=nightly \
    --build-arg TARGETARCH=amd64 --no-cache \
    . 

Run (mounting the template and pregen directory):

docker run --rm -it --name algod-pregen-run \
    -v /path/to/your/template.json:/etc/algorand/template.json \
    -v $(pwd)/pregen:/etc/algorand/keys \
    wwinder/algod:pregen-test

On my machine, I used a template.json with 10 million keys. Pregen took nearly 20 minutes. The network start was almost instantaneous.

[codecov]

Codecov Report

Merging #5735 (ddc6c7c) into master (527d2c5) will increase coverage by 1.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5735      +/-   ##
==========================================
+ Coverage   54.49%   55.57%   +1.08%     
==========================================
  Files         476      476              
  Lines       66853    66853              
==========================================
+ Hits        36429    37156     +727     
+ Misses      27876    27177     -699     
+ Partials     2548     2520      -28     

Files Changed	Coverage Δ
cmd/goal/network.go	15.95% <ø> (ø)

... and 41 files with indirect coverage changes

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[onetechnical]

The changes here look fine, but I didn't actually run them to verify the keys install properly.