Docker

Fixes for "Cannot connect to the Docker daemon at unix:///var/run/docker.sock" and "permission denied while trying to connect to the Docker daemon socket".

A container that stops right after docker run, or loops in "Restarting", almost always has a main process that exits or crashes. The exit code tells you why.

"Connection refused", "could not translate host name" or timeouts when one container calls another. Almost always a network, hostname or port mix-up.

A top-down routine for any "my container is not working" moment: status, logs, config, inside, network, resources. Work down the list and stop at the first thing that looks wrong.

The mental model behind every Docker command: images are read-only templates, containers are running instances of them, and volumes, networks and registries handle data, connectivity and distribution.

A pre-merge checklist for Dockerfiles: small, fast-building, secure and reproducible images.

Every Dockerfile instruction you need day to day, what it does, and the difference between the commonly confused pairs (CMD vs ENTRYPOINT, COPY vs ADD, ARG vs ENV).

Install Docker Engine and the Compose plugin on Ubuntu from Docker's official apt repository, then run docker without sudo.

Build in one stage with the full toolchain, then copy only the finished artifact into a small runtime image. Typical result: images 5-10x smaller with no compilers or source code inside.

Builds, pulls or containers fail with "no space left on device". Docker keeps old images, stopped containers, volumes, build cache and logs until you remove them.

Fix "Bind for 0.0.0.0:8080 failed: port is already allocated" and "address already in use" when starting a container.

Keep data alive beyond a container: named volumes for app data managed by Docker, bind mounts for host folders like source code, tmpfs for throwaway in-memory files.