Refactor command execution

[anbraten]

Currently commands are executed using a shell script injected into the container and executed.

woodpecker/pipeline/backend/common/script.go

Line 32 in faf6b33

entry = []string{"/bin/sh", "-c", "echo $CI_SCRIPT | base64 -d | /bin/sh -e"}

This could be removed to support some new possibilities:

• containers without /bin/sh, echo or base64
• know the command a log line is associated to WIP: Add command grouping and render closable log output per each command #3469
• ... (please add other valid points)

[anbraten]

Execute single command steps directly using docker exec and use the shell script for multiple commands

[anbraten]

Execute all commands directly using docker exec

[anbraten]

This options would lose the possibility to use sth like export TEST=true or cd tests/

[anbraten]

Use docker exec -it /bin/bash and inject commands one by one.

[anbraten]

Could be tricky to detect when the previous step finished to enter the next one using stdin

[anbraten]

Stick to script and add something like echo "::: start command 1" in between steps.