feat: build the magefile into a binary

This commit builds the magefile into a binary during the docker build
stage.
The Dockerfile is updated to include a multi-stage build where the first
stage compiles the binary and the second stage builds the final
cv-builder image.
The final image is now based on debian:buster instead of
golang:<VERSION>-buster which significantly reduces the image's size.

.dockerignore

@@ -1 +1,5 @@
 *
+!magefile.go
+!helpers/*
+!go.mod
+!go.sum

.gitlab/ci/docker-gitlab-ci.yml

@@ -21,9 +21,9 @@
 
 .docker-test-rules:
   rules:
-  - if: '$CI_COMMIT_BRANCH'
-    changes:
+  - changes:
     - "docker/Dockerfile"
+    - "*.go"
     when: always
   - if: '$CI_COMMIT_REF_NAME =~ /^[0-9]{4}(.[0-9]{2}){2}$/'
     when: never
@@ -39,9 +39,7 @@
 
 .docker-publish-rules:
   rules:
-  - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH && $CI_PROJECT_PATH == "dananglin/cv"'
-    changes:
-    - "docker/Dockerfile"
+  - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH && $CI_PROJECT_PATH == "dananglin/cv" && $CI_PIPELINE_SOURCE == "web" && $BUILD_DOCKER_IMAGE == "true"'
     when: always
 
 #------------------------------------------------#

docker/Dockerfile

@@ -1,11 +1,33 @@
-FROM golang:1.14.6-buster
+FROM golang:1.15.2-buster AS builder
 
-# Upgrade packages and install dependencies
+RUN git clone https://github.com/magefile/mage "${GOPATH}/src/mage"
+
+WORKDIR ${GOPATH}/src/mage
+
+RUN go run bootstrap.go
+
+COPY go.mod ${GOPATH}/cv-builder/
+COPY go.sum ${GOPATH}/cv-builder/
+COPY magefile.go ${GOPATH}/cv-builder/
+COPY helpers ${GOPATH}/cv-builder/helpers
+
+WORKDIR ${GOPATH}/cv-builder
+
+RUN mage -compile /usr/local/bin/cv-make
+
+FROM debian:buster
+
+COPY --from=builder /usr/local/bin/cv-make /usr/local/bin
+
+# Install dependencies
 RUN \
     apt-get update && \
     apt-get install -y --no-install-recommends \
         fonts-crosextra-carlito=20130920-1 \
         rsync=3.1.3-6 \
+        curl=7.64.0-4+deb10u1 \
+        aspell=0.60.7~20110707-6 \
+        aspell-en=2018.04.16-0-1 \
     && \
     apt-get clean autoclean && \
     apt-get autoremove -y && \