Mention dockerfile in documentation style guide.

[skieffer]

References to other Issues or PRs

Closes #22708

Brief description of what is fixed or changed

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Closes #22708 

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[github-actions]

Benchmark results from GitHub Actions

Lower numbers are good, higher numbers are bad. A ratio less than 1
means a speed up and greater than 1 means a slowdown. Green lines
beginning with + are slowdowns (the PR is slower then master or
master is slower than the previous release). Red lines beginning
with - are speedups.

Significantly changed benchmark results (PR vs master)

Significantly changed benchmark results (master vs previous release)

       before           after         ratio
     [907895ac]       [6632ed8b]
-         218±1ms        132±0.6ms     0.60  large_exprs.TimeLargeExpressionOperations.time_subs
-       225±0.5μs        104±0.2μs     0.46  matrices.TimeMatrixExpression.time_MatMul
-     14.2±0.02ms      7.64±0.02ms     0.54  matrices.TimeMatrixExpression.time_MatMul_doit
-      4.11±0.02s          313±2ms     0.08  polygon.PolygonArbitraryPoint.time_bench01
+     3.31±0.02ms      5.69±0.03ms     1.72  solve.TimeMatrixOperations.time_det(4, 2)
+     3.30±0.01ms      5.70±0.02ms     1.73  solve.TimeMatrixOperations.time_det_bareiss(4, 2)
+      37.5±0.3ms       69.9±0.2ms     1.87  solve.TimeMatrixSolvePyDySlow.time_linsolve(1)
+      38.3±0.7ms       70.0±0.5ms     1.83  solve.TimeMatrixSolvePyDySlow.time_solve(1)

Full benchmark results can be found as artifacts in GitHub Actions
(click on checks at the top of the PR).

[bertiewooster]

Nice. Perhaps add a link to Docker’s web site, so people know how to download it.