fix(version): copy the BUILD_VERSION file into the Docker image

[luislhl]

Acceptance Criteria

Our built Docker images should contain the BUILD_VERSION file, so they are able to report the injected version when running.

How to test

1. Create a BUILD_VERSION file in your local repo:

cat <<EOF > BUILD_VERSION                         
1.2.3
EOF

2. Build the image with docker build -t local/hathor-core .

3. Run it with docker run -it local/hathor-core run_node

The first log should contain the right version.

4. Remove the BUILD_VERSION file and build and run the image again. The version should now contain the -local prefix.

[codecov]

Codecov Report

Merging #576 (31cd945) into master (bfd6e04) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 31cd945 differs from pull request most recent head 0f056fc. Consider uploading reports for the commit 0f056fc to get more accurate results

@@           Coverage Diff           @@
##           master     #576   +/-   ##
=======================================
  Coverage   83.45%   83.46%           
=======================================
  Files         221      221           
  Lines       19144    19101   -43     
  Branches     2623     2618    -5     
=======================================
- Hits        15977    15942   -35     
+ Misses       2589     2583    -6     
+ Partials      578      576    -2     

see 8 files with indirect coverage changes