fix: Add exec to replace shell inside entrypoint with the actual binary

[hatemosphere]

What ❔

Replaces container shell with EN and local-env executables in corresponding Docker images to eg. properly handle termination signals.

More details in original PR: #76

Courtesy of https://github.com/voron
Thanks for the contribution, and sorry it took so long to review - we've been busy with FOSS'ing our repos.

Why ❔

#76 (comment)

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zk fmt and zk lint.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (38d66b1) 31.59% compared to head (2e3f19e) 31.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   31.59%   31.56%   -0.03%     
==========================================
  Files         477      477              
  Lines       25429    25429              
==========================================
- Hits         8034     8027       -7     
- Misses      17395    17402       +7     

see 2 files with indirect coverage changes

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