Dockerfile

FROM alpine:latest

ARG SOME_SECRET_SHIZ

RUN echo $SOME_SECRET_SHIZ > /tmp/secret
# Do something
RUN rm -f /tmp/secret

gistfile1.txt

$ docker build --squash --build-arg SOME_SECRET_SHIZ="$(cat ~/.secret)" -t myalpine .
Sending build context to Docker daemon 2.048 kB
Step 1/4 : FROM alpine:latest
 ---> a41a7446062d
Step 2/4 : ARG SOME_SECRET_SHIZ
 ---> Using cache
 ---> 66f605617eee
Step 3/4 : RUN echo $SOME_SECRET_SHIZ > /tmp/secret
 ---> Running in a0e33a90b962
 ---> 662f3d72b784
Removing intermediate container a0e33a90b962
Step 4/4 : RUN rm -f /tmp/secret
 ---> Running in c54ffdc3b923
 ---> aff3c0ccb67b
Removing intermediate container c54ffdc3b923
Successfully built aff3c0ccb67b

gistfile2.txt

# docker run -v /var/run/docker.sock:/run/docker.sock -ti -e TERM tomastomecek/sen

{
  "ContainerConfig": {
    "Entrypoint": null,
    "Image": "sha256:662f3d72b78454ea616780683b5089a12e87b4f0147a127796a69b3a69562f03",
    "Domainname": "",
    "OnBuild": [],
    "OpenStdin": false,
    "Tty": false,
    "Hostname": "9ac68176ac52",
    "ArgsEscaped": true,
    "Volumes": null,
    "StdinOnce": false,
    "Cmd": [
      "|1",
      "SOME_SECRET_SHIZ=oopsie",
      "/bin/sh",
      "-c",
      "rm -f /tmp/secret"
    ],
    "WorkingDir": "",
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
    ],
    "User": "",
    "AttachStdin": false,
    "AttachStdout": false,
    "AttachStderr": false,
    "Labels": {}
  },