Avoid wrapping long base64 encoded lines

When using combinations of username and password that result in longer
than 76 characters output base64 will insert a newline character.
In such a case kaniko will fail with an error message like this:
  /kaniko/.docker/config.json: invalid character '\n' in string literal

By removing the newlines from the base64 output everything
works as expected.

doc/ci/docker/using_kaniko.md

@@ -64,7 +64,7 @@ build:
     entrypoint: [""]
   script:
     - mkdir -p /kaniko/.docker
-    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64)\"}}}" > /kaniko/.docker/config.json
+    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64 | tr -d '\n')\"}}}" > /kaniko/.docker/config.json
     - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
   rules:
     - if: $CI_COMMIT_TAG
@@ -91,7 +91,7 @@ build:
     - mkdir -p /kaniko/.docker
     - |-
        KANIKOPROXYBUILDARGS=""
-       KANIKOCFG="{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64)\"}}}"
+       KANIKOCFG="{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64 | tr -d '\n')\"}}}"
        if [ "x${http_proxy}" != "x" -o "x${https_proxy}" != "x" ]; then
          KANIKOCFG="${KANIKOCFG}, \"proxies\": { \"default\": { \"httpProxy\": \"${http_proxy}\", \"httpsProxy\": \"${https_proxy}\", \"noProxy\": \"${no_proxy}\"}}"
          KANIKOPROXYBUILDARGS="--build-arg http_proxy=${http_proxy} --build-arg https_proxy=${https_proxy} --build-arg no_proxy=${no_proxy}"
@@ -120,7 +120,7 @@ store:
 ```yaml
 before_script:
   - mkdir -p /kaniko/.docker
-  - echo "{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64)\"}}}" > /kaniko/.docker/config.json
+  - echo "{\"auths\":{\"$CI_REGISTRY\":{\"auth\":\"$(echo -n ${CI_REGISTRY_USER}:${CI_REGISTRY_PASSWORD} | base64 | tr -d '\n')\"}}}" > /kaniko/.docker/config.json
   - |
     echo "-----BEGIN CERTIFICATE-----
     ...