Align a button to the right-end of a form

I am trying to get the below "sign in" button to be on the right end of my sign-in form but I cannot achieve it for some reason.

How I want it to look:

What it currently looks like:

What my frontend code looks like:

    <RuxContainer className="container">
      <div slot="header" className="header-container">
        <h3 className="first-header-item">LOGO</h3>
        <RuxClock timezone="Z"></RuxClock>
        <RuxMonitoringIcon status="normal"></RuxMonitoringIcon>
      </div>
      <div className="body-container">
        <form className="rux-form">
          <h2 className="body-sign-in">SIGN IN</h2>
          <h6>Measure your success in the Space Force!</h6>
          <div className="sign-in-inputs">
            <rux-input
              id="email"
              placeholder="Email@astro.com"
              label="Email"
              type="email"
              ruxblur="{handleValidation()}"
            ></rux-input>
            <rux-input id="pw" label="Password" type="password"></rux-input>
          </div>
          <div className="sign-in-helper-functions">
            <rux-checkbox class="checkbox">Remember me</rux-checkbox>
            <h2 className="forgot-password">Forgot Password?</h2>
          </div>
        </form>
        <rux-button className="sign-in-btn" type="submit">
          Sign in
        </rux-button>
      </div>
    </RuxContainer>

What my CSS looks like:

.header-container {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.body-container {
  display: flex;
  flex-direction: column;
  align-items: center;
}

.rux-form {
  width: 30%;
  max-width: 430px;
}

sign-in-btn {
  align-self: flex-end;
}

I am new to CSS so any help there would be greatly appreciated as well, thanks!

from Align a button to the right-end of a form