gitlab: introduce use of ccache for speeding up rebuilds

For any given job there is a high likelihood that ccache will be able to
reuse previously built object files. This will result in faster build
pipelines in later updates.

Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

@@ -13,6 +13,15 @@ stages:
 # Default native build jobs that are always run
 .native_build_default_job_template: &native_build_default_job_definition
   stage: native_build
+  cache:
+    paths:
+      - ccache/
+    key: "$CI_JOB_NAME"
+  before_script:
+    - mkdir -p ccache
+    - export CC="ccache gcc"
+    - export CCACHE_BASEDIR=${PWD}
+    - export CCACHE_DIR=${PWD}/ccache
   script:
     - mkdir build
     - cd build
@@ -31,6 +40,15 @@ stages:
 # Default cross build jobs that are always run
 .cross_build_default_job_template: &cross_build_default_job_definition
   stage: cross_build
+  cache:
+    paths:
+      - ccache/
+    key: "$CI_JOB_NAME"
+  before_script:
+    - mkdir -p ccache
+    - export CC="ccache ${ABI}-gcc"
+    - export CCACHE_BASEDIR=${PWD}
+    - export CCACHE_DIR=${PWD}/ccache
   script:
     - mkdir build
     - cd build