Optimize Dockerfile

Currently, changes to nearly any file in the repository will cause
`docker build` to have to reinitialize the python environment and
the Java environment.

This change reorders some of the commands and copies in dependent files
incrementally in order to maximize caching and make repeated test runs
move faster.

Dockerfile

@@ -19,22 +19,29 @@ RUN dnf -y update && \
         cargo \
     && dnf clean all
 
+# Install jsonschema-transpiler
 ENV PATH=$PATH:/root/.cargo/bin
 RUN cargo install jsonschema-transpiler --version 1.8.0
+
 # Configure git for testing
 RUN git config --global user.email "mozilla-pipeline-schemas@mozilla.com"
 RUN git config --global user.name "Mozilla Pipeline Schemas"
 
 WORKDIR /app
-COPY . /app
+
+# Install python dependencies
+COPY requirements.txt .
 RUN pip3 install -r requirements.txt
+
+# Install Java dependencies
+COPY pom.xml .
 RUN mvn dependency:copy-dependencies
 
-RUN rm -rf /app/release && \
-    mkdir /app/release && \
+COPY . /app
+
+RUN mkdir release && \
     cd release && \
     cmake .. && \
     make
 
-WORKDIR /app
 CMD pytest -v