Fix $PATH conflicts in .gitpod.Dockerfile (#1349)

- Use the correct user for gitpod Dockerfile.
- Remove unneeded packages (curl, rustc) from gitpod Dockerfile.

.gitpod.Dockerfile

@@ -1,7 +1,9 @@
 FROM gitpod/workspace-full
-USER root
-RUN apt-get update && apt-get install -y libssl-dev \
-    libxcb-composite0-dev \
-    pkg-config \
-    curl \
-    rustc
+
+USER gitpod
+
+RUN sudo apt-get update && \
+    sudo apt-get install -y \
+        libssl-dev \
+        libxcb-composite0-dev \
+        pkg-config