Update cheatsheets

docker-exec

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# docker exec
+
+> Execute a command on an already running Docker container.
+> More information: <https://docs.docker.com/engine/reference/commandline/exec/>.
+
+- Enter an interactive shell session on an already-running container:
+
+`docker exec --interactive --tty {{container_name}} {{/bin/bash}}`
+
+- Run a command in the background (detached) on a running container:
+
+`docker exec --detach {{container_name}} {{command}}`
+
+- Select the working directory for a given command to execute into:
+
+`docker exec --interactive --tty --workdir {{path/to/directory}} {{container_name}} {{command}}`
+
+- Run a command in background on existing container but keep `stdin` open:
+
+`docker exec --interactive --detach {{container_name}} {{command}}`
+
+- Set an environment variable in a running Bash session:
+
+`docker exec --interactive --tty --env {{variable_name}}={{value}} {{container_name}} {{/bin/bash}}`
+
+- Run a command as a specific user:
+
+`docker exec --user {{user}} {{container_name}} {{command}}`