add basic readme file

repository/README.md

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+# Alfresco Content Repository image
+
+## Description
+
+This Docker file is used to build an Alfresco Content Repository image.
+
+## Building the image
+
+Make sure all required artifacts are present in the build context `repository/`.
+iTYou can put them manually in the `repository/` folder (for example if that's a
+custom module of yours), or use the script `./scripts/fetch-artifacts.sh` to
+download them from Alfresco's Nexus.
+
+Then, you can build the image from the root of this git repository with the
+following command:
+
+```bash
+docker buildx bake repository
+```
+
+## Running the image
+
+### Alfresco repository configuration
+
+All preperties you would normally add in the alfresco-global.properties file can
+be added n the `JAVA_OPTS` environment variable to the container.
+
+For example, to set the database URL, you can use the following environment
+variable:
+
+```bash
+docker run -e JAVA_OPTS="-Ddb.url=jdbc:postgresql://postgres.domain.tld:5432/alfresco" \
+  alfresco-content-repository:mytag
+```
+
+> If the image is meant to be used with the Alfresco Content Services Helm
+> chart, you can use other [higher level means of
+> configuration](https://github.com/Alfresco/alfresco-helm-charts/blob/main/charts/alfresco-repository/docs/repository-properties.md).