fix: Avoid reference types in struct initialisation

Erlang/OTP 28 no longer allows regex literals
in struct defaults, due to non-serializable Reference types.

Instead, it sets it in the `new` function.

Author: jesse-c

lib/container.ex

@@ -23,7 +23,7 @@ defmodule Testcontainers.Container do
     labels: %{},
     auto_remove: false,
     container_id: nil,
-    check_image: ~r/.*/,
+    check_image: nil,
     network_mode: nil,
     reuse: false,
     force_reuse: false,
@@ -61,7 +61,7 @@ defmodule Testcontainers.Container do
   A constructor function to make it easier to construct a container
   """
   def new(image) when is_binary(image) do
-    %__MODULE__{image: image}
+    %__MODULE__{image: image, check_image: ~r/.*/}
   end
 
   @doc """

testcontainers-elixir-1.18-fix.md

@@ -0,0 +1,111 @@
+# Testcontainers Elixir 1.18 Compatibility Fix
+
+## GitHub Issue
+
+**Title:** Compilation error with Elixir 1.18: "cannot escape #Reference" in struct defaults
+
+**Description:**
+
+The testcontainers-elixir library fails to compile on Elixir 1.18+ due to a breaking change in how regex literals are handled in struct defaults.
+
+### Error Message
+```
+== Compilation error in file lib/container.ex ==
+** (ArgumentError) invalid value for struct field check_image, cannot escape #Reference<...>. 
+The supported values are: lists, tuples, maps, atoms, numbers, bitstrings, PIDs and remote functions in the format &Mod.fun/arity
+```
+
+### Root Cause
+Elixir 1.18 with Erlang/OTP 28 no longer allows regex literals to be used as struct default values because they contain internal `Reference` types that cannot be serialized during compilation. This is documented in the [Elixir 1.18 release notes](https://hexdocs.pm/elixir/changelog.html):
+
+> "Erlang/OTP 28 no longer allows regexes to be defined in the module body and interpolated into an attribute"
+
+### Current Code (lib/container.ex:26)
+```elixir
+defstruct [
+  check_image: ~r/.*/,  # This fails on Elixir 1.18+
+  # ...
+]
+```
+
+### Environment
+- Elixir: 1.18.4
+- Erlang/OTP: 28
+- testcontainers: 1.12.0
+
+## Fix Plan
+
+### 1. **Fix the struct definition** (`lib/container.ex:26`)
+```elixir
+# Change from:
+check_image: ~r/.*/,
+
+# To:
+check_image: nil,
+```
+
+### 2. **Update constructor functions** to handle regex initialization
+Look for functions like `new/1` or similar constructors and ensure they set the default regex:
+
+```elixir
+def new(opts \\ []) do
+  %__MODULE__{
+    check_image: Keyword.get(opts, :check_image, ~r/.*/),
+    # ... other fields
+  }
+end
+```
+
+### 3. **Search for other potential regex literals in struct defaults**
+Check if there are any other structs with similar issues:
+```bash
+grep -r "~r" lib/ | grep "defstruct"
+```
+
+### 4. **Update any container-specific modules**
+Check if `RedisContainer`, `PostgresContainer`, etc. have similar issues in their struct definitions.
+
+### 5. **Test the fix**
+- Compile the library: `mix compile`
+- Run existing tests: `mix test`
+- Verify Redis container specifically works
+
+### 6. **Create backwards compatibility**
+Ensure the fix works with both older and newer Elixir versions by testing that:
+- The `new/1` functions properly initialize the regex
+- Existing code using the containers continues to work
+- The default behavior remains the same (accepts any image by default)
+
+This is a minimal, targeted fix that maintains backward compatibility while resolving the Elixir 1.18 compilation issue.
+
+## Implementation Steps
+
+1. **Navigate to forked repository:**
+   ```bash
+   cd /Users/jesse/src/github.com/jesse-c/testcontainers-elixir
+   ```
+
+2. **Create feature branch:**
+   ```bash
+   git checkout -b fix/elixir-1.18-compatibility
+   ```
+
+3. **Make the changes:**
+   - Edit `lib/container.ex:26` to change `check_image: ~r/.*/,` to `check_image: nil,`
+   - Find and update constructor functions to initialize the regex at runtime
+   - Search for any other similar issues
+
+4. **Test the fix:**
+   ```bash
+   mix compile  # Should now work without errors
+   mix test     # Ensure existing functionality works
+   ```
+
+5. **Commit and push:**
+   ```bash
+   git add .
+   git commit -m "Fix Elixir 1.18 compatibility: move regex from struct default to runtime init"
+   git push origin fix/elixir-1.18-compatibility
+   ```
+
+6. **Create Pull Request** to the original testcontainers-elixir repository