feat: add support of maven_settings

README.md

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+# ci-image: MAVEN settings handling
+
+This image supports injecting a Maven `settings.xml` at container start via environment variables.
+
+Environment variables
+- `MAVEN_SETTINGS_BASE64` (preferred): base64-encoded contents of the `settings.xml`. If present, the entrypoint will decode it and write it to `~/.m2/settings.xml`.
+- `MAVEN_SETTINGS`: raw contents of the `settings.xml`. Used only if `MAVEN_SETTINGS_BASE64` is not set.
+
+Notes
+- The entrypoint will create `~/.m2` if it does not exist and set the `settings.xml` file to mode `644`.
+- Use `MAVEN_SETTINGS_BASE64` when your CI system has trouble preserving newlines or special characters in environment variables.
+
+Examples
+
+Build the image:
+
+```bash
+docker build -t my-ci-image .
+```
+
+Provide raw settings (works if your CI preserves newlines):
+
+```bash
+docker run --rm -e MAVEN_SETTINGS="$(cat ~/.m2/settings.xml)" my-ci-image bash -lc 'cat ~/.m2/settings.xml'
+```
+
+Provide base64-encoded settings (recommended to avoid newline issues):
+
+```bash
+docker run --rm -e MAVEN_SETTINGS_BASE64="$(base64 -w0 ~/.m2/settings.xml)" my-ci-image bash -lc 'cat ~/.m2/settings.xml'
+```
+
+GitHub Actions (example)
+
+If you store the plain `settings.xml` as a secret, encode it before setting a secret value (locally):
+
+```bash
+# encode locally then paste into GitHub secret value
+base64 -w0 ~/.m2/settings.xml
+```
+
+Then in your workflow use the secret as-is:
+
+```yaml
+env:
+  MAVEN_SETTINGS_BASE64: ${{ secrets.MAVEN_SETTINGS_BASE64 }}
+uses: docker://my-ci-image
+```
+
+If your CI runner can pass multiline env vars safely, you can instead set `MAVEN_SETTINGS` to the raw XML.