use xvfb-run

come on @jackfirth why must you torture me in this way

Dockerfile

@@ -1,9 +1,12 @@
 FROM racket/racket:8.0-full
-RUN raco pkg install --batch --auto --scope installation \
-         "https://github.com/jackfirth/rebellion.git"
-RUN raco pkg install --batch --auto --scope installation \
-         "https://github.com/jackfirth/resyntax.git"
-RUN raco pkg install --batch --auto --scope installation \
+RUN apt install xvfb
+# `resyntax` uses DrRacket for indenting, which requires a display, so we need to install it
+# under `xvfb-run`.
+# When/if this is closed, we can use racket-langserver instead (perhaps?):
+# https://github.com/jeapostrophe/racket-langserver/issues/45
+RUN xvfb-run raco pkg install --batch --auto --scope installation \
+         "https://github.com/jackfirth/rebellion.git" \
+         "https://github.com/jackfirth/resyntax.git" \
          "https://github.com/9999years/racket-package-resyntax-action.git#main"
 COPY entrypoint.sh /entrypoint.sh
 ENTRYPOINT ["/entrypoint.sh"]