build(docker): Optimize Docker image layers/caching & bump node to 14.15 (#776)

* refactor(docker): Combine RUN statements to reduce number of image layers

* Add --update && --no-cache to apk add command

* Combine RUN statements in first stage

* Consolidate COPY commands from BUILD_IMAGE

* Add docs & snap to .dockerignore

* Revert first COPY statement in 2nd stage

* Write committag.json in first stage & only do one COPY
Also pin base image Alpine version, and add Docker & GitHub Actions update monitoring via Dependabot

* Bump node to 14.15.4

* Change base image to node:14.15-alpine and remove Dependabot Docker monitoring

* Changes in response to PR comments

* Remove ARG/ENV statements from second build stage

.dockerignore

@@ -1,15 +1,23 @@
-node_modules
-.next
+**/*.md
+**/.gitkeep
+**/.vscode
+.all-contributorsrc
+.dockerignore
+.editorconfig
+.eslintrc.js
 .git
+.gitbook.yaml
 .gitconfig
 .gitignore
 .github
-.all-contributorsrc
-.editorconfig
+.next
 .prettierignore
-**/README.md
-**/.vscode
 config/db/db.sqlite3
 config/db/logs/overseerr.log
-Dockerfil**
-**.md
+Dockerfile*
+docker-compose.yml
+docs
+LICENSE
+node_modules
+snap
+stylelint.config.js