build: add documentation for ssl crypto provider feature flags

Author: kaitlin-smith-anaplan

.github/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
         with:
           tool: cargo-hack
       - name: Build
-        run: cargo hack build --each-feature --keep-going
+        run: cargo hack build --at-least-one-of ring,aws-lc-rs,ssl --feature-powerset --depth 2 --keep-going
 
   test:
     name: Test
@@ -62,7 +62,7 @@ jobs:
         with:
           tool: cargo-hack
       - name: Tests
-        run: cargo hack test --each-feature --clean-per-run
+        run: cargo hack test --at-least-one-of ring,aws-lc-rs,ssl --feature-powerset --depth 2 --clean-per-run
 
   fmt:
     name: Rustfmt check

docs/features/configuration.md

@@ -31,3 +31,13 @@ Configuration is fetched in the following order:
 1. `DOCKER_AUTH_CONFIG` environment variable, unmarshalling the string value from its JSON representation and using it as the Docker config.
 2. `DOCKER_CONFIG` environment variable, as an alternative path to the directory containing Docker `config.json` file.
 3. else it will load the default Docker config file, which lives in the user's home, e.g. `~/.docker/config.json`.
+
+## bollard, rustls and SSL Cryptography providers
+`testcontainers` uses [`bollard`](https://docs.rs/bollard/latest/bollard/) to interact with the Docker API. 
+
+`bollard` in turn has options provided by `rustls` to configure its SSL cryptography providers.
+
+The `testcontainers` feature flags to control this are as follows:
+* `ring` - use `rustls` with `ring` as the cryptography provider (default)
+* `aws-lc-rs` - use `rustls` with `aws-lc-rs` as the cryptography provider
+* `ssl` - use `rustls` with a custom cryptography provider configuration - see [bollard](https://docs.rs/bollard/latest/bollard/#feature-flags) and [rustls](https://docs.rs/rustls/latest/rustls/#cryptography-providers) documentation for more.