docker: Update docker config with node_exporter path.

[winder]

Summary

Fixes #5732

The default node exporter path is ./node_exporter. The docker container puts binaries on the path instead of using a fixed location, so this change simply fills in the absolute path.

Test Plan

Using the following config.json:

{
  "EnableMetricReporting": true
}

1. Control test:

docker run --rm -it \
      -p 9100:9100 \
      -v (pwd)/config.json:/etc/algorand/config.json \
      -e NETWORK=betanet \
      algorand/algod:3.18.0-stable

2. The metric endpoint does not work:

curl localhost:9100/metrics

3. Build container with this branch:

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

4. Run the container like above:

docker run --rm -it \
      -p 9100:9100 \
      -v (pwd)/config.json:/etc/algorand/config.json \
      -e NETWORK=betanet \
      wwinder/algod:metrics-fix

5. The metric endpoint works:

curl localhost:9100/metrics

[codecov]

Codecov Report

Merging #5762 (e43ca6d) into master (0972548) will increase coverage by 0.85%.
Report is 2 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5762      +/-   ##
==========================================
+ Coverage   54.59%   55.44%   +0.85%     
==========================================
  Files         473      473              
  Lines       66682    66682              
==========================================
+ Hits        36403    36973     +570     
+ Misses      27751    27190     -561     
+ Partials     2528     2519       -9     

see 44 files with indirect coverage changes

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