chore: create symbolic link to index.html.tmpl from index.html (#2284)

The existence of the index.html.tmpl tells Backstage to take that file and load it into memory, inject the needed appConfig (instead of writing
 it into a random *.chunk.js file, and then serve that file from memory so that we can set the readOnlyRootFilesystem: true option for
 the container.

https://issues.redhat.com/browse/RHIDP-5761

.rhdh/docker/Dockerfile

@@ -301,6 +301,11 @@ RUN chmod -R a+r ./dynamic-plugins/ ./install-dynamic-plugins.py; \
 RUN mkdir /opt/app-root/src/.npm
 RUN chown -R 1001:1001 /opt/app-root/src/.npm
 
+# The existence of the index.html.tmpl tells Backstage to take that file and load it into memory, inject the needed appConfig (instead of writing
+# it into a random *.chunk.js file, and then serve that file from memory so that we can set the readOnlyRootFilesystem: true option for
+# the container.
+RUN ln -s /opt/app-root/src/packages/app/dist/index.html /opt/app-root/src/packages/app/dist/index.html.tmpl
+
 # The fix-permissions script is important when operating in environments that dynamically use a random UID at runtime, such as OpenShift.
 # The upstream backstage image does not account for this and it causes the container to fail at runtime.
 RUN fix-permissions ./

docker/Dockerfile

@@ -236,6 +236,11 @@ COPY docker/install-dynamic-plugins.py docker/install-dynamic-plugins.sh ./
 RUN chmod -R a+r ./dynamic-plugins/ ./install-dynamic-plugins.py; \
   chmod -R a+rx ./install-dynamic-plugins.sh;
 
+# The existence of the index.html.tmpl tells Backstage to take that file and load it into memory, inject the needed appConfig (instead of writing
+# it into a random *.chunk.js file, and then serve that file from memory so that we can set the readOnlyRootFilesystem: true option for
+# the container.
+RUN ln -s /opt/app-root/src/packages/app/dist/index.html /opt/app-root/src/packages/app/dist/index.html.tmpl
+
 # The fix-permissions script is important when operating in environments that dynamically use a random UID at runtime, such as OpenShift.
 # The upstream backstage image does not account for this and it causes the container to fail at runtime.
 RUN fix-permissions ./