Add a couple of precompilation statements

[giordano]

main (da1ccc1):

% julia +1.7 --startup-file=no --project=/tmp -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false);
  2.771236 seconds (10.20 M allocations: 540.632 MiB, 7.27% gc time, 100.00% compilation time)

% julia +1.8 --startup-file=no --project=/tmp -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false);
  2.598736 seconds (8.77 M allocations: 448.371 MiB, 7.11% gc time, 99.77% compilation time)

% julia +1.9 --startup-file=no --project=/tmp -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false);
  2.492451 seconds (4.97 M allocations: 332.907 MiB, 6.76% gc time, 99.71% compilation time)

This PR:

% julia +1.7 --startup-file=no --project=/tmp -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false);
  0.046847 seconds (1.96 k allocations: 120.734 KiB, 100.02% compilation time)

% julia +1.8 --startup-file=no --project=/tmp -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false);
  1.136760 seconds (45.95 k allocations: 2.163 MiB, 99.34% compilation time)

% julia +1.9 --startup-file=no --project=/tmp -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false);
  0.007143 seconds (8.53 k allocations: 526.817 KiB)

This makes this package much more useful for quick usage. Fix #76 (except with v1.8, but it's still faster than without any precompilation statements).

[codecov]

Codecov Report

Base: 79.71% // Head: 79.61% // Decreases project coverage by -0.11% ⚠️

Coverage data is based on head (422029f) compared to base (da1ccc1).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
- Coverage   79.71%   79.61%   -0.11%     
==========================================
  Files           6        6              
  Lines         567      569       +2     
==========================================
+ Hits          452      453       +1     
- Misses        115      116       +1     

Impacted Files	Coverage Δ
src/Sandbox.jl	72.80% <0.00%> (-0.65%)	⬇️
src/Docker.jl	86.13% <0.00%> (+0.10%)	⬆️

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